Solid oxide fuel cell system for producing variable generated power based on power demand

ABSTRACT

The present invention is to provide a solid oxide fuel cell capable of improving the overall energy efficiency. The present invention is directed to a solid oxide fuel cell and comprising: a fuel cell module; a fuel supply device; a combustion chamber for burning excess fuel and heating; a heat storing material, a power demand detecting sensor; a temperature detection device, and a control device for controlling so that the fuel utilization rate is high when generated power is large, and also for changing output power at a delay to the fuel supply rate; whereby the control device comprises a stored heat amount estimating circuit, and when it is estimated that a utilizable heat amount has accumulated in the heat storing material, the fuel supply rate is reduced so that the fuel utilization rate increases vs. the same generated power.

TECHNICAL FIELD

This application is a 371 application of PCT/JP2011/072331 having aninternational filing date of Sep. 29, 2011, which claims priority to JP2010-218367 filed Sep. 29, 2010, JP 2011-077954 filed Mar. 31, 2011, JP2011-077955 filed Mar. 31, 2011, and JP 2011-079465 filed Mar. 31, 2011,the entire contents of which are incorporated herein by reference.

The present invention pertains to a solid oxide fuel cell, and moreparticularly to a solid oxide fuel cell for generating variableelectrical power in response to power demand.

BACKGROUND ART

Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate atrelatively high temperatures in which, using an oxide ion-conductingsolid electrolyte as electrolyte, with electrodes attached to both sidesthereof, fuel gas is supplied to one side thereof and an oxidizer (air,oxygen, or the like) is supplied to the other side thereof.

In such SOFCs, steam or CO₂ is produced by the reaction between oxygenions passed through an oxide ion-conducting solid electrolyte and fuel,thereby generating electrical and thermal energy. The electrical energyis removed from the SOFC, where it is used for various electricalpurposes. On the other hand, thermal energy is used to raise thetemperature of the fuel, the reformer, the water, the oxidant, and thelike.

JP 2010-92836 (Patent Document 1) sets forth a fuel cell device. Thisfuel cell device is a solid oxide fuel cell of the type which changesgenerated power in response to power demand; operation is disclosed inwhich a fuel utilization rate is reduced more which it operates in thelow load range than when it operates in the range where the powergeneration load is high. That is, in Patent Document 1, a proportion offuel used for power generation with respect to the entire supplied fuelis reduced when generated power is in a low state, but on the otherhand, fuel used to heat the fuel cell module and not used to generateelectricity is not greatly reduced, and a large fraction of the fuel isused to heat the fuel cell module so that the fuel cell module isoperated in a thermally independent manner at a temperature at whichpower generation can occur is maintained.

Specifically, when the module operates in the range where the generatedpower is low, heat generated in the fuel cell unit in association withelectrical generation declines. As a result, the temperature inside thefuel cell module tends to decline. Therefore, if the fuel utilizationrate is maintained at a certain level even when the module operates inthe range where the power generation is low, a decline of thetemperature inside the fuel cell module is induced, and it becomesdifficult to maintain the temperature at which power can be generated.Therefore, fuel used to heat the fuel cell module is increased in orderto operate in a thermally independent manner, even sacrificing the fuelutilization rate.

In the fuel cell device set forth in JP 2010-92836, in order to resolvethese problems the fuel utilization rate is reduced when the moduleoperates in the low load range where electrical generation is small,thereby preventing an excessive temperature drop in the fuel cell modulewhile stably maintaining a certain high temperature state.

JP 2010-205670 (Patent Document 2), meanwhile, sets forth a method foroperating a fuel cell system and fuel cell. In this fuel cell, a valueis acquired which represents s sum of the fuel cell electrical loads,and the fuel utilization rate is controlled based on the acquired value.Control of the fuel utilization rate is performed by estimating the fuelcell temperature based on the value representing a sum of the fuel cellelectrical loads. Then, the fuel utilization rate is controlled based onthe estimated temperature. The fuel cell can therefore be operated in athermally independent manner without the use of a temperature sensor.Also, when the value representing a sum of the electrical loads is equalto or greater than a predetermined value, a correction mode corrects thefuel utilization rate to a value equal to or greater than a referencevalue at which the fuel cell can be thermally independently operated. Insuch cases, because the temperature of the fuel cell has already beenhigh, there is surplus heat in the fuel cell, and thermally independentoperation is maintained even if the fuel utilization rate is correctedto a value equal to or greater than the reference value at whichthermally independent operation is possible. System efficiency of thefuel cell system is by this means improved.

-   Patent Document 1 JP 2010-92836-   Patent Document 2 JP 2010-205670

SUMMARY OF THE INVENTION Problems the Invention Seeks to Resolve

As noted above, however, when the fuel utilization rate is reduced inthe fuel cell in the manner set forth in JP 2010-92836, although thermalindependence may be assured, fuel is increased that does not contributeto electrical generation. Therefore, when an operation is performed inwhich the fuel utilization rate is reduced, the problem arises that theoverall energy efficiency of the solid oxide fuel cell is reduced. Sincethe overall energy efficiency reduces more as the module operates longerat the reduced fuel utilization rate state, the solid oxide fuel cell(SOFC), considered advantageous in the fuel efficiency over the polymermembrane fuel cells (PEFC), loses the advantage.

In particular, the solid oxide fuel cell is considered mainly used inresidences, where there is a certain time period in a day, such as anight time at which the residents are in sleep, when the fuel cell isoperated in a low power generation state, the overall energy efficiencyof the solid oxide fuel cell should be low when used in residents. It isneeded to develop a solid oxide fuel cell technology capable ofoperating the solid oxide fuel at a high fuel utilization rate and athigh efficiency even in such low electrical generation states.

In the fuel cell set forth in JP 2010-205670, the fuel utilization rateis controlled based on a value representing a sum of fuel cellelectrical loads, thereby eliminating the temperature sensor. When thevalue representing electrical loads is equal to or greater than apredetermined value, the fuel utilization rate is increased. Therefore,in a state in which the value is equal to or greater than apredetermined value, the fuel supply rate is reduced, thus raising thesystem efficiency.

In the fuel cell set forth in JP 2010-205670, however, since the fuelutilization rate is controlled based on the value representing a sum ofelectrical loads without the use of a temperature sensor, the problemarises that the system efficiency cannot be sufficiently improved. Also,In particular, in the fuel cell system that is operated in such a mannerthat the electrical load (generated power, etc.) is increased with adelay after the fuel supply rate is increased, the efficiency cannot besufficiently improved. That is, in the fuel cell systems in which theelectrical load (generated power, etc.) is increased with a delay afterthe fuel supply rate is increased, a large gap develops between thevalue representing the fuel supply amount and the value representing asum of the electrical loads, making it difficult to accurately estimatethe surplus heat in the fuel cell system, and thus impeding efficiencyimprovement. When power demand suddenly decreases, the generated currentshould be swiftly decreased to prevent a reverse power flow of thecurrent When it happens, a difference of the physical responsivenessinevitably causes a reduction of the fuel supply rate to be delayedafter a reduction of the generated current. Thereby, a large gapdevelops between the fuel supply amount and a sum of the electricalloads.

Specifically, in the invention set forth in JP 2010-205670, the heat(joule heat) generating during electrical generation by the fuel cellsis estimated based on an amount of generated electricity, therebyestimating surplus heat. By contrast, the heat generated by a delay inelectrical generation by the fuel cell device is combustion heat createdby the combustion of excess fuel not used for electrical generation.Therefore, the amount of heat accumulated during the startup processcannot be estimated using the technology disclosed in JP 2010-205670.

When the fuel utilization rate is raised based on inaccurately estimatedsurplus heat in the fuel cell system, there is a risk that the fuel cellstack temperature could suddenly drop, and the cells are damaged.Furthermore, if a delay time is shortened when the electrical load isincreased in order to reduce a gap between the value representing thefuel supply amount and the value representing a sum of the electricalloads, there is a risk that the current will be extracted from the fuelcells before sufficient fuel has reached each of the fuel cells, causingfuel cut-off. Conversely, under a condition that the temperature of thefuel cell stack rises, when a small amount of utilizable surplus heat isused, the problem arises that the temperature of the fuel cell stackrises excessively.

The object of the present invention is to provide an extremely practicalsolid oxide fuel cell capable of improving the overall energy efficiencywhile maintaining thermal independence and operating stably.

It is also a purpose of this invention to provide a solid oxide fuelcell capable of fully utilizing accumulated heat while reliablymaintaining the thermal independence, and of avoiding excessivetemperature rises.

Means for Resolving the Problem

In order to solve the above-described problem, the present invention isto provide a solid oxide fuel cell for generating power variable inresponse to power demand, comprising: a fuel cell module for generatingelectricity using supplied fuel; a fuel supply device configured tosupply fuel to the fuel cell module; a generating oxidant gas supplydevice configured to supply oxidant gas for electrical generation to thefuel cell module; a combustion section for burning remaining excess fuelsupplied by the fuel supply device and not utilized for electricalgeneration, and heating the interior of the fuel cell module; a heatstoring material for storing heat generated within the fuel cell module;a power demand detection device configured to detect power demand; atemperature detection device configured to detect the temperature of thefuel cell module; and a control device configured to control, based onthe power demand detected by the power demand detection device, suchthat the fuel utilization rate is high when generated power is large,and the fuel utilization rate is low when generated power is small, andalso for changing the power actually output from the fuel cell modulewith a delay after changing the fuel supply rate in response to changesin power demand; wherein the control device includes a stored heatestimating circuit for estimating surplus heat stored in the heatstoring material, based on the temperature detected by the temperaturedetection device, and the control device reduces the fuel supply rate sothat the fuel utilization rate increases relative to the same generatedpower when it is determined that a usable amount of heat is stored inthe heat storing material, in comparison with cases in which it isdetermined that a usable amount of heat has not been stored.

In the present invention thus constituted, the fuel supply device andgenerating oxidant gas device respectively supply fuel and generatingoxidant gas to the fuel cell module. The fuel cell module generateselectricity using the supplied fuel and generating oxidant gas, whileexcess fuel not used for generation is combusted in the combustionsection, and the resulting heat is stored in the heat storage material.Based on a power demand detected using a power demand detection device,the control device controls the fuel supply device so that the fuelutilization rate is high when the generated power is large, and the fuelutilization rate is low when the generated power is small. In addition,the control device changes the power actually output from the fuel cellmodule with a delay after causing the fuel supply rate to change inresponse to changes in power demand. Based on the temperature detectedby the temperature detection device, the stored heat estimating circuitestimates the amount of surplus heat stored in the heat storingmaterial. When it is determined by the stored heat estimating circuitthat a usable amount of heat is stored in the heat storing material, thecontrol device reduces the fuel supply rate so that the fuel utilizationrate relative to the same generated power is higher than in the casewhen a usable amount of heat is not stored.

Generally speaking, when power generated in a solid oxide fuel cell issmall, heat generating therefrom drops, resulting in that the fuel cellmodule temperature drops. The fuel utilization rate is therefore reducedat times power generated is low, and fuel not used for electricalgeneration is combusted to heat the fuel cell module and preventexcessive temperature drops. In particular, in solid oxide fuel cells ofa type in which the reformer is disposed within the fuel cell module,endothermic reactions occur inside the reformer, further causing thetemperature to drop. In the present invention constituted as describedabove, when it is determined that a usable amount of heat is stored inthe heat storing material, the fuel supply rate is reduced so that thefuel utilization rate increases. The thermal independence of the solidoxide fuel cell is thus maintained, and the overall energy efficiency ofthe solid oxide fuel cell is improved, while excessive temperature dropsare avoided.

In the present invention constituted as described above, the stored heatamount is estimated based on the temperature detected by the temperaturedetection device. Therefore, the control device can accurately estimatethe accumulated heat amount even if the output power is changed with adelay after the fuel supply rate is changed. This enables the amount ofheat stored in the heat storing material to be fully utilized whilesecurely avoiding the risk of sudden temperature drops in the fuel cellmodule. In addition, in fuel cells of the type which changes outputpower with a delay after changing the fuel supply rate, frequentincreases and decreases of output power cause a great deal of excessfuel use and risk excessive temperature rises inside the fuel cellmodule, but the present invention constituted as described above enablesan accurate estimation of the stored heat caused by the excess fuelarising in this manner. In general, a cooling medium is inserted intothe fuel cell module to suppress the excessive temperature rise causedby excess fuel, but the present invention enables an accurate estimationof the heat amount stemming from excess fuel, the effective use of whichenables excessive temperature rises to be suppressed. The amount ofcooling medium inserted for the purpose of lowering the temperature cantherefore be reduced, and the overall energy efficiency of the solidoxide fuel cell can be improved.

In the present invention, the stored heat estimating circuit preferablyestimates the amount of surplus heat stored in the heat storing materialbased on a detected temperature history.

In the present invention thus constituted, the stored heat estimatingcircuit estimates the stored heat amount based on the temperaturedetection history. Therefore, the stored heat amount can be moreaccurately estimated compared to estimating the stored heat amount fromrecent detected temperatures alone. The amount of surplus heat stored inthe heat storing material can therefore be fully used.

In the present invention, the control device preferably greatlyincreases the fuel utilization rate as the stored heat amount estimatedby the stored heat estimating circuit increases.

In the present invention thus constituted, the stored surplus heat isused in large quantity when the estimated stored heat amount is large,and not much stored heat is used when the stored heat amount is small.Therefore, stored heat can be more effectively utilized, and the risk oftemperature drops can be reliably avoided.

In the present invention, the control device preferably determines thefuel utilization rate based on predetermined conditions in addition tothe stored heat amount estimated by the stored heat estimating circuit,and the power demand.

In the present invention thus constituted, conditions other than thestored surplus heat amount and the power demand are added for thedetermination of the fuel utilization rate. Therefore, stored heat canbe appropriately utilized in response to the operation state of the fuelcell module.

In the present invention, the control device preferably changes the fuelutilization rate relative to changes in the estimated stored surplusheat amount much more in the range in which the estimated stored surplusheat amount estimated by the stored heat estimating circuit is largethan in the range where the estimated stored surplus heat amount issmall.

In the present invention thus constituted, when the estimated storedsurplus heat amount is large, a large amount of stored surplus heat isutilized and excessive temperature rises can be avoided, whereas whenthe estimated stored surplus heat amount is small, stored surplus heatcan be used a little at a time, and overcooling can be avoided.

In the present invention, the stored heat estimating circuit preferablyestimates the stored surplus heat amount based on recent changes in adetected temperature in addition to the detected temperature history,and the control device changes the fuel utilization rate much more whenthe change in a recent detected temperature is large than when it issmall.

In the present invention thus constituted, stored surplus heat isestimated by recent detected temperature changes in addition to thehistory of detected temperatures. Therefore, stored surplus heat can beaccurately estimated based on the history, and fuel cell moduletemperature changes in which the heat capacity is large and for which itnot easy to change the trend once the change has started can beresponded to with agility, preventing excessive temperature rises andtemperature drops.

In the present invention, the control device preferably changes the fuelutilization rate over a wider scale in the range where the generatedpower is small than in the range where the generated power is large.

In the present invention thus constituted, the fuel utilization rate ischanged over a larger scale in the range where the generated power issmall than in the range where the generated power is large. The risk ofsudden temperature drops can therefore be reduced, and by greatlyutilizing stored surplus heat in the range in which the generated poweris small where there is an ample margin for improving the fuelutilization rate, the energy efficiency can be effectively increased.Under normal control, as well, in the range where the generated power islarge where the fuel utilization rate is high, there is only a smallmargin for improving the fuel utilization rate, and thus by notsignificantly utilizing stored surplus heat in this interval, the energyefficiency in the range where the generated power is small can beimproved by utilizing stored heat accumulated in the range where thegenerated power is large.

In the present invention, the control device preferably reduces changesincreasing the fuel utilization rate more after the fuel cell module hasdegraded than before the fuel cell module had degraded.

In the present invention thus constituted, changes raising the fuelutilization rate are reduced after the fuel cell module degradesTherefore, advancement of degradation can be prevented by creating acooling tendency in a fuel cell module which has risen in temperatureduring electrical generation due to degradation.

In the present invention, the stored heat estimating circuit preferablyestimates the stored heat amount based on a value representing a sum ofadd/subtract values determined based on the detected temperature, andbased on a differential value between new detected temperatures and pastdetected temperatures.

In the present invention thus constituted, a stored surplus heat amountis estimated based on the value representing a sum of add/subtractvalues and on differential values. Therefore, the stored surplus heatamount can be appropriately estimated by simple calculation, based uponwhich the fuel utilization rate can be appropriately set.

In the present invention, the stored heat estimating circuit preferablyestimates stored surplus heat by adding or subtracting the add/subtractvalues determined based on the detected temperature and otherpredetermined conditions.

In the present invention thus constituted, the add/subtract values forestimating stored surplus heat are determined based on predeterminedconditions in addition to the detected temperature. Therefore, factorsother than the temperature affecting stored heat can be appropriatelyreflected in the estimate values.

In the present invention, the stored heat estimating circuit preferablyuses positive and negative values for the add/subtract values based onthe detected temperature and the generated power.

In the present invention thus constituted, increases or decreases of theamount of stored surplus heat are estimated not only by the detectedtemperature but also the generated power is also referred, permitting amore accurate estimation of stored surplus heat amounts.

In the present invention, the stored heat estimating circuit preferablychanges the estimated stored surplus heat value more quickly asgenerated power increases.

In the present invention thus constituted, the estimated stored surplusheat value is more quickly changed as the generated power increases.Therefore, an estimation of stored surplus heat becomes matched moreclosely to the actual conditions can be achieved.

In the present invention, the stored heat estimating circuit preferablyestimates the stored surplus heat amount based on a basic estimatedvalue calculated based on a detected temperature history, and on a quickresponse estimated value calculated based on the rate of change of thedetected temperature during an interval shorter than the history overwhich the basic estimated value is calculated.

In the present invention thus constituted, the power output can bechanged after a safe time for dispersing fuel is secured by changing theoutput power with a delay following changes to the fuel supply rate,thereby avoiding the risk that cells in the fuel cell module are damageddue to fuel cut-off. Also, excess fuel increases due to the delay inoutputting electrical power, and the excess fuel heats the inside of thefuel cell module. When the fuel cell module has high thermal insulationcharacteristics and it is necessary to perform excessive load-followingcontrol under which output power is frequently raised and lowered, itoccurs that excessive temperature rises are caused inside the fuel cellmodule due to the accumulation of heat caused by the excess fuel. Ingeneral, the amount of generating oxidant gas supplied as a coolingmedium is increased in order to lower the temperature inside the fuelcell module. But since the temperature drops caused by the insertion ofcooling medium is achieved by discharging a usable surplus heat amountin the fuel cell module together with the exhaust, the overall energyefficiency drops. The solid oxide fuel cell of the present inventionsuppresses excessive temperature rises by reducing the introduced fuelsupply rate in such a way that excessively accumulated surplus heat isutilized while maintaining thermal independence, and furthermoresimultaneously achieves a high fuel utilization rate operation. Toachieve this, the stored heat estimating circuit estimates storedsurplus heat based on the detected temperature. Therefore, estimationcan be carried out by accurately considering the effects of stored heatarising from the excess fuel produced by delaying the output of powerrelative to the supply of fuel. Excessive temperature rises occurringwhen there is excessive load following can thus be reliably preventedwhile the energy efficiency is raised.

Furthermore, in the present invention thus constituted, the stored heatestimating circuit estimates the stored surplus heat amount based on abasic estimated value calculated based on a detected temperaturehistory, and on a quick response estimated value calculating the rate ofchange of the detected temperature during an interval shorter than thehistory over which the basic estimated value is calculated. Therefore,by estimating the stored surplus heat amount which will serve as a baseusing the history of the extremely gradually changing detectedtemperature, and using the rapid response estimated value, the storedheat amount estimated value can promptly respond to the trends of thechange in the detected temperature. So by using an estimate for thestored surplus heat amount, which is inherently difficult to estimate,the stored surplus heat can be effectively utilized while avoiding therisk of excessive temperature rises and temperature drops, without theuse of any special detection devices.

In the present invention, multiple temperature detection devices may beused and wherein the stored heat estimating circuit estimates the amountof stored surplus heat accumulated in the heat storing material based onthe past history of multiple temperatures detected by the multipletemperature detection devices; and based on the stored surplus heatamount estimated by the stored heat estimating circuit and on the powerdemand detected by the power demand detection device, the control devicedetermines a fuel utilization rate such that for the same generatedpower, the fuel utilization rate increases as the estimated storedsurplus heat amount increases, and controls the fuel supply device basedon this fuel utilization rate.

In the present invention thus constituted, the stored heat estimatingcircuit estimates the stored surplus heat amount accumulated in the heatstoring material based on multiple detected temperatures detected bymultiple temperature detection devices, and the control device controlsthe fuel utilization rate so that for the same generated power, the fuelutilization rate increases as the estimated stored heat amountincreases. By utilizing the stored heat accumulated in the heat storingmaterial, the fuel supply rate can be reduced and the overall energyefficiency of the solid oxide fuel cell improved.

Also, because the stored surplus heat amount is estimated based ondetected temperature, the estimated stored surplus heat amount fullyreflects the amount of excess fuel, and the estimated stored surplusheat amount is highly reliable. In particular, the stored surplus heatamount can be accurately estimated when generated power is increased,even in the type of fuel cell in which generated power is increased witha delay. This enables the amount of surplus heat stored in the heatstoring material to be fully utilized while avoiding the risk oftemperature drops and the like in the fuel cell module.

Furthermore, in the present invention thus constituted, the storedsurplus heat amount is estimated based on multiple temperatures detectedby the multiple temperature detection devices. Therefore, the risk ofpartial temperature drops or temperature rises within the fuel cellmodule can be avoided, and the stored surplus heat amount can be fullyutilized.

In the present invention, the stored heat estimating circuit preferablyestimates the stored surplus heat amount by treating the hightemperature among the multiple detected temperatures as a high weightingfactor when increasing the estimated stored heat amount to increase thefuel utilization rate, and by treating the low temperature among themultiple detected temperatures as a high weighting factor whendecreasing the estimated stored heat amount to decrease the fuelutilization rate.

In the present invention thus constituted, the stored surplus heatamount estimation is performed using the high detected temperature as ahigh weighting factor when increasing the fuel utilization rate, and thelow detected temperature is treated as a high weighting factor whendecreasing the fuel utilization rate. Therefore, the stored surplus heatamount can be estimated by using the detected temperatures on therespective safe sides relative to excessive temperature rises andtemperature drops.

In the present invention, the multiple temperature detection devices arepreferably disposed so that the reformer temperature and the fuel cellstack temperature are respectively reflected, and the control devicesuppresses the rise in the fuel utilization rate when the reformertemperature is at or below a predetermined usage-suppressing reformertemperature.

In the present invention thus constituted, increases in the fuelutilization rate are suppressed when the reformer temperature is at orbelow a predetermined use-suppressing reformer temperature. Therefore,degradation of reformer performance can be prevented when the reformeris excessively cooled by endothermic reactions or the like within thereformer. Damage to the fuel cell stack associated with degradedreformer performance can also be avoided.

In the present invention, the fuel cell stack is preferably constitutedof multiple fuel cell units arrayed in approximately a rectangular form.One of the multiple temperature detection devices is disposed to reflectthe temperature of the individual fuel cell unit positioned at a vertexof the rectangle. One of the multiple temperature detection devices isdisposed to reflect the temperature of the individual fuel cell unitpositioned at the midpoint between two vertices of the rectangle; andthe control device suppresses increases in the fuel utilization ratewhen the temperature of an individual fuel cell unit positioned at arectangle vertex is at or below a predetermined usage-suppressing cellunit temperature.

In the present invention thus constituted, increases in the fuelutilization rate are suppressed when the temperature of an individualfuel cell unit positioned at a rectangle vertex is at or below thetemperature of a predetermined usage-suppressing cell unit temperature.Therefore, increases in the fuel utilization rate are suppressed basedon the temperature of an individual fuel cell unit at a low temperaturewithin the fuel cell stack, and damage caused by excessive cooling of aportion of the cell units can be prevented.

Effect of the Invention

Using the solid oxide fuel cell of the present invention, the overallenergy efficiency can be improved while maintaining the thermalindependence and achieving stable operation.

Also, According to the solid oxide fuel cell of the present invention,the stored surplus heat amount can be fully utilized while reliablymaintaining the thermal independence, and excessive temperature risescan be avoided.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: An overview diagram showing a fuel cell device according to anembodiment of the present invention.

FIG. 2: A front elevation cross section showing a fuel cell module in afuel cell device according to an embodiment of the present invention.

FIG. 3: A sectional diagram along line in FIG. 2.

FIG. 4: A partial cross section showing a fuel cell unit in a fuel celldevice according to an embodiment of the present invention.

FIG. 5: An oblique view showing a fuel cell stack in a fuel cell deviceaccording to an embodiment of the present invention.

FIG. 6: A block diagram showing a fuel cell device according to anembodiment of the present invention.

FIG. 7: A timing chart showing the operation when a fuel cell device isstarted, according to an embodiment of the present invention.

FIG. 8: A timing chart showing the operation when a fuel cell device isstopped, according to an embodiment of the present invention.

FIG. 9: A graph showing the relationship between output current and fuelsupply rate in the solid oxide fuel cell of the first embodiment of thepresent invention.

FIG. 10: A graph showing the relationship between output current andamount of heat produced by supplied fuel in the solid oxide fuel cell ofthe first embodiment of the present invention.

FIG. 11: A control flow chart of the fuel supply rate in the solid oxidefuel cell of the first embodiment of the present invention.

FIG. 12: A stored heat amount estimate table used to estimate the amountof heat accumulated in a heat storing material in the solid oxide fuelcell of the first embodiment of the present invention.

FIG. 13: A graph of the stored heat amount estimate table in FIG. 12.

FIG. 14: A graph showing the value of a first modifying coefficientrelative to output current in the solid oxide fuel cell of the firstembodiment of the present invention.

FIG. 15: A graph showing the value of a second modifying coefficientrelative to output current in the solid oxide fuel cell of the firstembodiment of the present invention.

FIG. 16: A flowchart for changing correction amounts when the fuel cellmodule has degraded.

FIGS. 17(a) and 17(b): A graph schematically showing changes in powerdemand over a day in a typical residence.

FIG. 18: A graph showing the value of a current modifying coefficient ina variant example of the first embodiment of the present invention.

FIG. 19: A graph schematically showing the relationship between changesin power demand, fuel supply rate, and current actually extracted from afuel cell module.

FIG. 20: A graph showing an example of the relationship betweengenerating air supply rate, water supply rate, fuel supply rate, andcurrent actually extracted from a fuel cell module.

FIG. 21: A flowchart showing the order in which generating air supplyrate, water supply rate, and fuel supply rate are determined based ondetected temperature Td.

FIG. 22: A graph showing appropriate fuel cell stack temperature vs.generating current.

FIG. 23: A graph showing fuel utilization rate determined according toaccumulated value.

FIG. 24: A graph showing the range of fuel utilization rates which canbe determined relative to each generating current.

FIG. 25: A graph showing air utilization rates determined according toaccumulated value.

FIG. 26: A graph showing the range of air utilization rates which can bedetermined relative to each generating current.

FIG. 27: A graph for determining water supply rates vs. a determined airsupply utilization rate.

FIG. 28: A graph showing appropriate fuel cell module generating voltagevs. generating current.

FIG. 29: A flowchart showing the procedure for limiting the range ofpower produced by the fuel cell module in a second embodiment of thepresent invention.

FIG. 30: A map showing current limits vs. generating current anddetected temperature.

FIG. 31: A timing chart showing an example of the effect of the secondembodiment of the present invention.

FIG. 32: A graph showing an example of the relationship betweentemperature inside the fuel cell module and maximum generatable power.

FIG. 33: A flow chart showing a procedure for calculating a firstadd/subtract value based on temperatures detected by multipletemperature sensors.

FIG. 34: A flow chart showing the procedure for calculating anadd/subtract value according to a variant example of the secondembodiment of the present invention.

FIG. 35: A flow chart showing the procedure for calculating anadd/subtract value according to a variant example of the secondembodiment of the present invention.

EMBODIMENTS OF THE INVENTION

Next, referring to the attached drawings, we discuss a solid oxide fuelcell (SOFC) according to embodiments of the present invention.

FIG. 1 is an overview diagram showing a solid oxide fuel cell (SOFC)according to an embodiment of the present invention. As shown in FIG. 1,the solid oxide fuel cell (SOFC) of this embodiment of the presentinvention comprises a fuel cell module 2 and an auxiliary unit 4.

The fuel cell module 2 comprises a housing 6. A sealed space 8 is formedwithin the housing 6 and surrounded by an insulating material 7. A fuelcell assembly 12 for carrying out the electrical generating reactionbetween fuel gas and oxidizer (air) is disposed in a generating chamber10 in the lower portion of the sealed space 8. The fuel cell assembly 12comprises ten fuel cell stacks 14 (see FIG. 5), and each of the fuelcell stacks 14 comprises 16 fuel cell units 16 (see FIG. 4). Thus, thefuel cell assembly 12 has 160 fuel cell units 16, all of which areserially connected.

A combustion chamber 18 is formed above the aforementioned generatingchamber 10 in the sealed space 8 of the fuel cell module 2; excess fuelgas and residual oxidizer (air) not used in the electrical generationreaction are burned in this combustion chamber 18 and produce exhaustgas.

A reformer 20 for reforming fuel gas is disposed at the top of thecombustion chamber 18; the reformer 20 is heated by the heat of residualgas combustion to a temperature at which the reforming reaction can takeplace. Furthermore, an air heat exchanger 22 is disposed on the top ofthis reformer 20 for receiving heat from the reformer 20 and heating airso as to restrain temperature drops in the reformer 20.

Next, the auxiliary unit 4 comprises a pure water tank 26 for holdingwater from a municipal or other water supply source 24 and filtering itinto pure water, and a water flow regulator unit 28 (a “water pump” orthe like driven by a motor) for regulating the flow rate of watersupplied from the reservoir tank. The auxiliary unit 4 is furtherfurnished with a gas shutoff valve 32 for shutting off the fuel gassupply from a fuel supply source 30 such as municipal gas or the like,and a fuel flow regulator unit 38 (a “fuel pump” or the like driven by amotor) for regulating the flow rate of fuel gas. Furthermore, anauxiliary unit 4 comprises an electromagnetic valve 42 for shutting offair serving as an oxidizer and supplied from an air supply source 40, areforming air flow regulator unit 44 and oxidant gas supply device 45(an “air blower” or the like driven by a motor) for regulating air flowrate, a first heater 46 for heating reforming air supplied to thereformer 20, and a second heater 48 for heating generating air suppliedto the generating chamber. This first heater 46 and second heater 48 areprovided in order to efficiently raise the temperature at startup, andmay be omitted.

Next, a hot-water producing device 50 supplied with exhaust gas isconnected to the fuel cell module 2. Municipal water from a water supplysource 24 is supplied to this hot-water producing device 50; this wateris turned into hot water by the heat of the exhaust gas, and is suppliedto a hot water reservoir tank in an external water heater, not shown.

A control box 52 for controlling the amount of fuel gas supplied, etc.is connected to the fuel cell module 2.

Furthermore, an inverter 54 serving as an electrical power extractionunit (electrical power conversion unit) for supplying electrical powergenerated by the fuel cell module to the outside is connected to thefuel cell module 2.

Next, the internal structure of the solid oxide fuel cell (SOFC) fuelcell module of this embodiment of the present invention is explainedusing FIGS. 2 and 3. FIG. 2 is a side elevation sectional diagramshowing a fuel cell module in a solid oxide fuel cell (SOFC) accordingto an embodiment of the present invention; FIG. 3 is a sectional diagramalong line of FIG. 2.

As shown in FIGS. 2 and 3, starting from the bottom in the sealed space8 within the fuel cell module 2 housing 6, a fuel cell assembly 12, areformer 20, and an air heat exchanger 22 are arranged in sequence, asdescribed above.

A pure water guide pipe 60 for introducing pure water on the upstreamend of the reformer 20, and a reform gas guide pipe 62 for introducingthe fuel gas and reforming air to be reformed, are attached to thereformer 20; a steam generating section 20 a and a reforming section 20b are formed in sequence starting from the upstream side within thereformer 20, and the reforming section 20 b is filled with a reformingcatalyst. Fuel gas and air blended with the steam (pure water)introduced into the reformer 20 is reformed by the reforming catalystused to fill in the reformer 20. Appropriate reforming catalysts areused, such as those in which nickel is imparted to the surface ofaluminum spheres, or ruthenium is imparted to aluminum spheres.

A fuel gas supply line 64 is connected to the downstream end of thereformer 20; this fuel gas supply line 64 extends downward, and thenfurther extends horizontally within a manifold formed under the fuelcell assembly 12. Multiple fuel supply holes 64 b are formed on thebottom surface of a horizontal portion 64 a of the fuel gas supply line64; reformed fuel gas is supplied into the manifold 66 from these fuelsupply holes 64 b.

A lower support plate 68 provided with through holes for supporting theabove-described fuel cell stack 14 is attached at the top of themanifold 66, and fuel gas in the manifold 66 is supplied into the fuelcell unit 16.

Next, an air heat exchanger 22 is provided over the reformer 20. Thisair heat exchanger 22 comprises an air concentration chamber 70 on theupstream side and two air distribution chambers 72 on the downstreamside; this air concentration chamber 70 and the distribution chambers 72are connected using six air flow conduits 74. Here, as shown in FIG. 3,three air flow conduits 74 form a set (74 a, 74 b, 74 c, 74 d, 74 e, 74f); air in the air concentration chamber 70 flows from each set of theair flow conduits 74 to the respective air distribution chambers 72.

Air flowing in the six air flow conduits 74 of the air heat exchanger 22is pre-heated by rising combustion exhaust gas from the combustionchamber 18.

Air guide pipes 76 are connected to each of the respective airdistribution chambers 72; these air guide pipes 76 extend downward,communicating at the bottom end side with the lower space in thegenerating chamber 10, and introducing preheated air into the generatingchamber 10.

Next, an exhaust gas chamber 78 is formed below the manifold 66. Asshown in FIG. 3, an exhaust gas conduit 80 extending in the verticaldirection is formed on the insides of the front surface 6 a and the rearsurface 6 b which form the faces in the longitudinal direction of thehousing 6; the top inside of the exhaust gas conduit 80 communicateswith the space in which the air heat exchanger 22 is disposed, and thebottom end side communicates with the exhaust gas chamber 78. An exhaustgas discharge pipe 82 is connected at approximately the center of thebottom surface of the exhaust gas chamber 78; the downstream end of thisexhaust gas discharge pipe 82 is connected to the above-described hotwater producing device 50 shown in FIG. 1.

As shown in FIG. 2, an ignition device 83 for starting the combustion offuel gas and air is disposed on the combustion chamber 18.

Next, referring to FIG. 4, we discuss the fuel cell unit 16. FIG. 4 is apartial section showing a solid oxide fuel cell (SOFC) fuel cell unitaccording to an embodiment of the present invention.

As shown in FIG. 4, the fuel cell unit 16 comprises a fuel cell 84 andinternal electrode terminals 86, respectively connected to therespective terminals at the top and bottom of the fuel cell 84.

The fuel cell 84 is a tubular structure extending in the verticaldirection, furnished with a cylindrical internal electrode layer 90, onthe inside of which is formed a fuel gas flow path 88, a cylindricalexternal electrode layer 92, and an electrolyte layer 94 between theinternal electrode layer 90 and the external electrode layer 92. Thisinternal electrode layer 90 is a fuel electrode through which fuel gaspasses, and is a (−) pole, while the external electrode layer 92 is anair electrode which contacts the air, and is a (+) pole.

The internal electrode terminals 86 attached at the top end and bottomends of the fuel cell device 16 have the same structure, therefore wewill here discuss specifically the internal electrode terminal 86attached at the top and side. The top portion 90 a of the internalelectrode layer 90 comprises an outside perimeter surface 90 b and topend surface 90 c, exposed to the electrolyte layer 94 and the externalelectrode layer 92. The internal electrode terminal 86 is connected tothe outer perimeter surface of the internal electrode layer 90 through aconductive seal material 96, and is electrically connected to theinternal electrode layer 90 by making direct contact with the top endsurface 90 c of the internal electrode layer 90. A fuel gas flow path 98communicating with the internal electrode layer 90 fuel gas flow path 88is formed at the center portion of the internal electrode terminal 86.

The internal electrode layer 90 is formed, for example, from at leastone of a mixture of Ni and zirconia doped with at least one type of rareearth element selected from Ca, Y, Sc, or the like; or a mixture of Niand ceria doped with at least one type of rare earth element; or anymixture of Ni with lanthanum gallate doped with at least one elementselected from among Sr, Mg, Co, Fe, or Cu.

The electrolyte layer 94 is formed, for example, from at least one ofthe following: zirconia doped with at least one type of rare earthelement selected from among Y, Sc, or the like; ceria doped with atleast one type of selected rare earth element; or lanthanum gallatedoped with at least one element selected from among Sr or Mg.

The external electrode layer 92 is formed, for example, from at leastone of the following: lanthanum manganite doped with at least oneelement selected from among Sr or Ca; lanthanum ferrite doped with atleast one element selected from among Sr, Co, Ni, or Cu; lanthanumcobaltite doped with at least one element selected from among Sr, Fe,Ni, or Cu; silver, or the like.

Next we discuss the fuel cell stack 14, referring to FIG. 5. FIG. 5 is aperspective view showing the fuel cell stack in a solid oxide fuel cell(SOFC) according to an embodiment of the present invention.

As shown in FIG. 5, the fuel cell stack 14 comprises sixteen fuel cellunits 16; the top inside and bottom inside of these fuel cell units 16are respectively supported by a lower support plate 68 and upper supportplate 100. Through holes 68 a and 100 a, through which the internalelectrode terminal 86 can penetrate, are provided on this lower supportplate 68 and upper support plate 100.

In addition, a collector 102 and an external terminal 104 are attachedto the fuel cell unit 16. This collector 102 is integrally formed by afuel electrode connecting portion 102 a, which is electrically connectedto the internal electrode terminal 86 attached to the internal electrodelayer 90 serving as the fuel electrode, and by an air electrodeconnecting portion 102 b, which is electrically connected to the entireexternal perimeter of the external electrode layer 92 serving as the airelectrode. The air electrode connecting portion 102 b is formed of avertical portion 102 c extending vertically along the surface of theexternal electrode layer 92, and multiple horizontal portions 102 dextending in the horizontal direction from this vertical portion 102 calong the surface of the external electrode layer 92. The fuel electrodeconnecting portion 102 a extends linearly in an upward or downwarddiagonal direction from the vertical portion 102 c of the air electrodeconnecting portion 102 b toward the internal electrode terminals 86positioned in the upper and lower directions on the fuel cell unit 16.

Furthermore, the internal electrode terminals 86 at the top and bottomends of the two fuel cell units 16 positioned at the end of the fuelcell stack 14 (at the front and back sides on the left edge in FIG. 5)are respectively connected to the external terminals 104. These externalterminals 104 are connected to the external terminals 104 (not shown) atthe ends of the adjacent fuel cell stack 14, and as described above, allof the 160 fuel cell units 16 are connected in series.

Next, referring to FIG. 6, we discuss the sensors attached to the solidoxide fuel cell (SOFC) according to the present embodiment. FIG. 6 is ablock diagram showing a solid oxide fuel cell (SOFC) according to anembodiment of the present invention.

As shown in FIG. 6, a solid oxide fuel cell device 1 comprises a controlsection 110; an operating device 112 provided with operating buttonssuch as “ON” or “OFF” for user operation, a display device 114 fordisplaying various data such as a generator output value (Watts), and anotification device 116 for issuing warnings during abnormal states andthe like are connected to this control section 110. This notificationdevice 116 may be connected to a remote control center to inform thecontrol center of abnormal states.

Next, signals from the various sensors described below are input to thecontrol section 110.

First, a flammable gas detection sensor 120 detects gas leaks and isattached to the fuel cell module 2 and the auxiliary unit 4.

The purpose of the flammable gas detection sensor 120 is to detectleakage of CO in the exhaust gas, which is meant to be exhausted to theoutside via the exhaust gas conduit 80, into the external housing (notshown) which covers the fuel cell module 2 and the auxiliary unit 4.

A water reservoir state detection sensor 124 detects the temperature andamount of hot water in a water heater (not shown).

An electrical power state detection sensor 126 detects current, voltage,and the like in the inverter 54 and in a distribution panel (not shown).

A generator air flow detection sensor 128 detects the flow rate ofgenerator air supplied to the generating chamber 10.

A reforming air flow rate sensor 130 detects the rate of reforming airflow supplied to the reformer 20.

A fuel flow rate sensor 132 detects the flow rate of fuel gas suppliedto the reformer 20.

A water flow rate sensor 134 detects the flow rate of pure watersupplied to the reformer 20.

A water level sensor 136 detects the water level in pure water tank 26.

A pressure sensor 138 detects pressure on the upstream side outside thereformer 20.

An exhaust temperature sensor 140 detects the temperature of exhaust gasflowing into the hot water producing device 50.

As shown in FIG. 3, a generating chamber temperature sensor 142 isdisposed on the front surface side and rear surface side around the fuelcell assembly 12, and detects the temperature around the fuel cell stack14 in order to estimate the temperature of the fuel cell stack 14 (i.e.,of the fuel cell 84 itself).

A combustion chamber temperature sensor 144 detects the temperature incombustion chamber 18.

An exhaust gas chamber temperature sensor 146 detects the temperature ofexhaust gases in the exhaust gas chamber.

A reformer temperature sensor 148 detects the temperature of thereformer 20 and calculates the reformer 20 temperature from the intakeand exit temperatures on the reformer 20.

If the solid oxide fuel cell (SOFC) is placed outdoors, the outsidetemperature sensor 150 detects the temperature of the outsideatmosphere. Sensors to detect outside atmospheric humidity and the likemay also be provided.

Signals from these various sensor types are sent to the control section110; the control section 110 sends control signals to the water flowregulator unit 28, the fuel flow regulator unit 38, the reforming airflow regulator unit 44, and the oxidant gas supply device 45 based ondata from the sensors, and controls the flow rates in each of theseunits.

Next, referring to FIG. 7, we discuss the operation of a solid oxidefuel cell (SOFC) according to the present embodiment at the time ofstart up. FIG. 7 is a timing chart showing the operations of a solidoxide fuel cell (SOFC) according to an embodiment of the presentinvention at the time of start up.

At the beginning, in order to warm up the fuel cell module 2, operationstarts in a no-load state, i.e., with the circuit which includes thefuel cell module 2 in an open state. At this point current does not flowin the circuit, therefore the fuel cell module 2 does not generateelectricity.

First, reforming air is supplied from the reforming air flow regulatorunit 44 to the reformer 20 on the fuel cell module 2. At the same time,generating air is supplied from the oxidant gas supply device 45 to theair heat exchanger 22 of the fuel cell module 2, and this generating airreaches the generating chamber 10 and the combustion chamber 18.

Immediately thereafter, fuel gas is also supplied from the fuel flowregulator unit 38, and fuel gas into which reform area is blended passesthrough the reformer 20, the fuel cell stack 14, and the fuel cell unit16 to reach the combustion chamber 18.

Next, ignition is brought about by the ignition device 83, and fuel gasand air (reforming air and generating air) supplied to the combustionchamber 18 is combusted. This combustion of fuel gas and air producesexhaust gas; the generating chamber 10 is warmed by this exhaust gas,and when the exhaust gas rises into the sealed space 8 of the fuel cellmodule 2, the fuel gas, which includes reforming air in the reformer 20is warm, as is the generating air inside the air heat exchanger 22.

At this point, fuel gas into which reform area is blended is supplied tothe reformer 20 by the fuel flow regulator unit 38 at the reforming airflow regulator unit 44, therefore the partial oxidation reformingreaction POX given by Expression (1) proceeds. This partial oxidationreforming reaction POX is an exothermic reaction, and therefore hasfavorable starting characteristics. The fuel gas whose temperature hasrisen is supplied from the fuel gas supply line 64 to the bottom of thefuel cell stack 14, and by this means the fuel cell stack 14 is heatedfrom the bottom, and the combustion chamber 18 is also heated by thecombustion of the fuel gas and air, so that the fuel stack 14 is alsoheated from above, enabling as a result an essentially uniform rise intemperature in the vertical direction of the fuel cell stack 14. Eventhough the partial oxidation reforming reaction POX is progressing, theongoing combustion reaction between fuel gas and air is continued in thecombustion chamber 18.C_(m)H_(n) +xO₂ →aCO₂ +bCO+CH₂  (1)

When the reformer temperature sensor 148 detects that the reformer 20has reached a predetermined temperature (e.g. 600° C.) after the startof the partial oxidation reforming reaction POX, a pre-blended gas offuel gas, reforming air, and steam is applied to the reformer 20 by thewater flow regulator unit 28, the fuel flow regulator unit 38, and thereforming air flow regulator unit 44. At this point an auto-thermalreforming reaction ATR, which makes use of both the aforementionedpartial oxidation reforming reaction POX and the steam reformingreaction SR described below, proceeds in the reformer 20. Thisauto-thermal reforming reaction ATR can be internally thermallybalanced, therefore the reaction proceeds in a thermally independentfashion inside the reformer 20. In other words, when there is a largeamount of oxygen (air), heat emission by the partial oxidation reformingreaction POX dominates, and when there is a large amount of steam, theendothermic steam reforming reaction SR dominates. At this stage, theinitial stage of startup has passed and some degree of elevatedtemperature has been achieved within the generating chamber 10,therefore even if the endothermic reaction is dominant, no major drop intemperature will be caused. Also, the combustion reaction continueswithin the combustion chamber 18 even as the auto-thermal reformingreaction ATR proceeds.

When the reformer temperature sensor 148 detects that the reformer 20has reached a predetermined temperature (e.g., 700° C.) following thestart of the auto-thermal reforming reaction ATR shown as Expression(2), the supply of reforming air by the reforming air flow regulatorunit 44 is stopped, and the supply of steam by the water flow regulatorunit 28 is increased. By this means, a gas containing no air and onlycontaining fuel gas and steam is supplied to the reformer 20, where thesteam reforming reaction SR of Expression (3) proceeds.C_(m)H_(n) +xO₂ +yH₂O→aCO₂ +bCO+CH₂  (2)C_(m)H_(n) +xH₂O→aCO₂ +bCO+CH₂  (3)

This steam reforming reaction SR is an endothermic reaction, thereforethe reaction proceeds as a thermal balance is maintained with thecombustion heat from the combustion chamber 18. At this stage, the fuelcell module is in the final stages of startup, therefore the temperaturehas risen to a sufficiently high level within the generating chamber 10so that no major temperature dropped is induced in the generatingchamber 10 even though an endothermic reaction is proceeding. Also, thecombustion reaction continues to proceed in the combustion chamber 18even as the steam reforming reaction SR is proceeding.

Thus, after the fuel cell module 2 has been ignited by the ignitiondevice 83, the temperature inside the generating chamber 10 graduallyrises as a result of the partial oxidation reforming reaction POX, theauto-thermal reforming reaction ATR, and the steam reforming reaction SRwhich proceed in that sequence. Next, when the temperature inside thegenerating chamber 10 and the temperature of the fuel cell 84 reaches apredetermined generating temperature which is lower than the ratedtemperature at which the cell module 2 can be stably operated, thecircuit which includes the fuel cell module 2 is closed, electricalgeneration by the fuel cell module 2 begins, and current then flows tothe circuit. Generation of electricity by the fuel cell module 2 causesthe fuel cell 84 itself to emit heat, such that the temperature of thefuel cell 84 rises. As a result, the rated temperature at which the fuelcell module 2 is operated becomes, for example, 600° C.-800° C.

Following this, a quantity of fuel gas and air greater than thatconsumed by the fuel cell 84 is applied in order to maintain the ratedtemperature and continue combustion inside the combustion chamber 18.Generation of electricity by the high reform-efficiency steam reformingreaction SR proceeds while electricity is being generated.

Next, referring to FIG. 8, we discuss the operation upon stopping thesolid oxide fuel cell (SOFC) of the present embodiment. FIG. 8 is atiming chart showing the operations which occur upon stopping the solidoxide fuel cell (SOFC) of the present embodiment.

As shown in FIG. 8, when stopping the operation of the fuel cell module2, the fuel flow regulator unit 38 and the water flow regulator unit 28are first operated to reduce the quantity of fuel gas and steam beingsupplied to the reformer 20.

When stopping the operation of the fuel cell module 2, the quantity ofgenerating air supplied by the reforming air flow regulator unit 44 intothe fuel cell module 2 is being increased at the same time that thequantity of fuel gas and steam being supplied to the reformer 20 isbeing reduced; the fuel cell assembly 12 and the reformer 20 are aircooled to reduce their temperature. Thereafter, when the temperature ofthe generating chamber drops to, for example, 400° C., supply of thefuel gas and steam to the reformer 20 is stopped, and the steamreforming reaction SR in the reformer 20 ends. Supply of the generatingair continues until the temperature in the reformer 20 reaches apredetermined temperature, e.g. 200° C.; when the predeterminedtemperature is reached, the supply of generating air from the oxidantgas supply device 45 is stopped.

Thus in the present embodiment the steam reforming reaction SR by thereformer 20 and cooling by generating air are used in combination,therefore when the operation of the fuel cell module 2 is stopped, thatoperation can be stopped relatively quickly.

Next, referring to the FIGS. 9 through 17, we discuss the control of asolid oxide fuel cell 1 according to a first embodiment of the presentinvention.

FIG. 9 is a graph showing the relationship between the output currentand the fuel supply rate in the solid oxide fuel cell 1 of the firstembodiment of the present invention.

FIG. 10 is a graph showing the relationship between the output currentand the amount of heat produced by the supplied fuel in the solid oxidefuel cell 1 of the first embodiment of the present invention.

First, as shown by the solid line in FIG. 9, the solid oxide fuel cell 1of the present embodiment is capable of changing its electrical outputat or below the rated output power of 700 W (output current 7 A) inresponse to power demand. The fuel supply rate (L/min) necessary tooutput the required powers are set by a Basic Fuel Supply Table andshown along the solid line in FIG. 9. A control section 110, whichserves as a control device, determines a fuel supply rate based on thefuel supply rate table in response to the power demand detected by anelectrical power state detecting sensor 126, which serves as a powerdemand detection device. The fuel flow regulator unit 38 serving as afuel supply device is controlled based on the fuel supply rate.

The supply rate of fuel needed for generation of the requiredelectricity changes in proportion to the output power (output current),but as shown by the solid line in FIG. 9, the fuel supply rate set inthe basic fuel supply rate table do not change in proportion to theoutput currents. This is because when the fuel supply rate is reduced inproportion to a reduction of the output power, it becomes impossible tomaintain the fuel cell units 16 in the fuel cell module 2 at thetemperature at which the fuel cell units 16 are capable of generatingelectricity. Therefore, in the present embodiment, the basic fuel supplytable is set to achieve a fuel utilization rate of approximately 70%when the fuel cell units 16 generate a large power or output a currentin the range near 7 A, and is set to achieve a fuel utilization rate ofapproximately 50% when the fuel cell units 16 generate a small power oroutput a current in a range near 2 A. Thus, by reducing the fuelutilization rate in the range where the generated power is small andburning fuel not used for generation of electricity to heat the reformer20 and other members, the temperature of the fuel cell units 16 can besuppressed from falling, and the temperature sufficiently higher forgeneration of electricity can be maintained in the fuel cell module 2.

However, a reduction of the fuel utilization rate causes an increase offuel not contributing to electrical generation, so the energy efficiencyof the solid oxide fuel cell 1 declines in the range where the generatedpower is small. In the solid oxide fuel cell 1 of the presentembodiment, a fuel table change circuit 110 a built into the controlsection 110 changes or corrects the fuel supply rate set in the basicfuel supply table in response to predetermined conditions and reducesthe fuel supply rate down to those as shown along the dotted line inFIG. 9, so that the fuel utilization rate in the range where thegenerated power is small is raised. The energy efficiency of the solidoxide fuel cell 1 is thus improved.

FIG. 10 is a graph schematically showing the relationship between theoutput current generated when the fuel is supplied according to thebasic fuel supply table and the amount of heat generated from thesupplied fuel in the solid oxide fuel cell 1 of the present embodiment.As shown by the dot-and-dash line in FIG. 10, the amount of heat neededto make the fuel cell module 2 thermally autonomous and to operate thefuel cell module 2 stably increases linearly with an increase of theoutput current. The solid line in FIG. 10 shows the heat amountgenerated when fuel is supplied according to the basic fuel supplytable. In this embodiment, the necessary heat amount indicated by thedot-and-dash line and the amount of heat generated from the fuelsupplied in accordance with the basic fuel supply table shown by thesolid line are approximately matched in the region where the outputcurrent is below 5 A, which corresponds to a medium power generation.

Furthermore, in the region where the output current is above 5 A, theheat amount shown by the solid line which is generated at the fuelsupply rates according to the basic fuel supply table is greater thanthe heat amount shown by the dot-and-dash line, which is the minimumheat required for thermal independence. The amount of surplus heatdefined between the solid line and the dotted line is accumulated in theinsulating material 7 serving as a heat storing material. There is alsoa correlation between the output current from the solid oxide fuel cell1 and the temperature of the fuel cell units 16 in the fuel cell module2 which is measured when this current is being output in a steady state.Since the temperature of the fuel cell units 16 should be raised inorder to increase the output current, the temperature of the fuel cellunits 16 is high when the output current is high. In the presentembodiment, an output current of 5 A corresponds to approximately 633°C., which is the heat storage temperature Th. Therefore in the solidoxide fuel cell 1 of the present embodiment, a larger amount of heat isaccumulated in the insulating material 7 when the output current is 5 Aand the heat storage temperature Th=approximately 633° C. or above.

The heat storage temperature Th is set to a temperature corresponding to500 W (an output current of 5 A), which is larger than the 350 Wrepresenting the midpoint value of the generated power range of 0 W-700W. In the rage where the output current is 5 A or below, the heat amountgenerated at fuel supply rates defined in the basic fuel supply table isset to be approximately the same as the minimum heat amount required forthermal independence (the heat amount in the basic fuel supply table isslightly higher). Therefore as shown by the dotted line example in FIG.10, the heat amount falls short of the heat amount needed for thermalindependence when the fuel supply rate from the basic fuel supply tableis corrected and reduced.

In the present embodiment, as described below, a correction is made inthe range where the generated power is mall in order to temporarilyreduce the fuel supply rate set by the basic fuel supply table and raisethe fuel utilization rate. At the same time, the balance of the heatamount caused by the reduction of the fuel supply rate defined in thebasic fuel supply table is made up for by using the heat amount storedin the insulating material 7 which is accumulated while the fuel cellmodule 2 is operating in a temperature region above the heat storagetemperature Th. Note that in the present embodiment, because the heatcapacity of the insulating material 7 is extremely high, the heat amountaccumulated in the insulating material 7 can be used over a period ofmore than 2 hours when the fuel cell module 2 is operating in the rangewhere the generated power is small after operating to generate highpower for a predetermined time, and the fuel utilization rate can beraised by performing a correction to reduce the fuel supply rate duringthis interval.

Also, in the present embodiment, in the rage where the output current is5 A and the heat storage temperature Th=approximately 633° C. or above,the basic fuel supply table is set so that a greater amount of heat isaccumulated in the insulating material 7, but in the rage where theoutput current is 5 A or above, the basic fuel supply table may also beset to generate heat approximately equal to the minimum heat requiredfor thermal independence (the heat amount in the basic fuel supply tableis slightly higher). That is, in a range where the generated power islarge, the operating temperature of the fuel cell module 2 is higherthan when generated power is small Therefore, even if the fuel supplyrate is set to generate the minimum heat required for thermalindependence, the heat amount usable when the generated power is smallcan be accumulated in the insulating material 7. In this embodiment, inwhich the fuel supply rate is set high when the generated power is high,the necessary amount of surplus heat can be reliably accumulated in theinsulating material 7 during the short evening time period when powerdemand is at peak.

Next, referring to the FIGS. 11 through 17, we discuss the specificcontrol of a solid oxide fuel cell 1 according to the first embodimentof the present invention.

FIG. 11 is a control flow chart of the fuel supply rate in the solidoxide fuel cell of the first embodiment of the present invention. FIG.12 is a stored surplus heat estimation table used to estimate the amountof surplus heat accumulated in the insulating material 7. FIG. 13 is agraph of the stored surplus heat estimation table. FIG. 14 is a graphshowing the value of a first modifying coefficient exhibited relative tothe output current. FIG. 15 is a graph showing the value of a secondmodifying coefficient exhibited relative to the output current.

The flow chart shown in FIG. 11 is executed at predetermined timeintervals in the control section 110 while the solid oxide fuel cell 1generates power. First, as step S1 in FIG. 11, integration process isexecuted based on the stored heat estimating table shown in FIG. 12. Theintegrated value Ni calculated in step S1 is, as described below, avalue which will serve as an index for a usable amount of surplus heataccumulated in the insulating material 7 or the like, and lies between 0and 1.

Next, in step S2, a judgment is made as to whether the integrated valueNi calculated in step S1 is a 0. If the integrated value Ni is 0, theprocess proceeds to step S3. If other than 0, it proceeds to step S4.

When the integrated value is a 0, it is estimated that surplus heatsufficiently usable has not been accumulated in the insulating material7. Therefore, in step S3, the fuel supply rate is determined by thecontrol section 110 as set in the basic fuel supply table. The controlsection 110 sends a signal to the fuel flow regulator unit 38, and thedetermined fuel supply rate is supplied to the fuel cell module 2.Therefore, in this case, no correction is executed to raise the fuelutilization rate even when the generated power is small. After step S3,one iteration of the process in the flow chart of FIG. 11 is completed.

In step S4, on the other hand, a rate change to be made to the fuelsupply rate defined in the basic fuel supply table is determined basedon the integrated value Ni. That is, when the integrated value Ni is 1,the fuel supply rate is reduced most and thus the fuel utilization rateis improved. The amount of reduction in the fuel supply rate decreasesas the integrated value Ni decreases.

Next, in step S5, the first modifying coefficient is determined based onthe graph shown in FIG. 14. As shown in FIG. 14, the first modifyingcoefficient is 1 in the range where the output current is small, andgoes to 0 when output current exceeds 4.5 A. For example, in the rangewhere the generated power is small, a correction is executed to reducethe fuel supply rate by using the amount of surplus heat accumulated inthe insulating material 7, and the fuel utilization rate is improved,whereas no correction is executed in the range where the generated poweris large. This is because during the operation in which the generatedpower is large, a sufficiently high fuel utilization rate can beachieved using the basic fuel supply table, and when the generated poweris large, it is difficult to utilize the heat stored in the insulatingmaterial 7 because of the high temperature inside the fuel cell module2.

Next, in step S6, the second modifying coefficient is determined basedon the graph shown in FIG. 15. As shown in FIG. 15, the second modifyingcoefficient is 0.5 in the region where the output current is 1 A orbelow, and grows linearly in the range where the output current is 1 to1.5 A. It takes a value of 1 when the output current is 1.5 A or above.In other words, in the range where the generated power is 150 W orbelow, where a change to the fuel utilization is suppressed, the fuelsupply rates defined in the basic fuel supply table is small So there isa risk of causing damage to the fuel cell units 16 when a largereduction is made to those fuel supply rates. Also, by keeping low theamount of correction to be made to the basic fuel supply table, theamount of surplus heat accumulated in the insulating material 7 can beused a little at a time, making it possible to utilize the storedsurplus heat over a long period. Therefore, the second modifyingcoefficient reduces the amount of correction to the basic fuel supplytable more as the generated power becomes smaller, so that it functionsas a change period extension circuit for extending the time periodduring which the basic fuel supply table is changed or corrected. Thechange period extension circuit operates to further prolong the timeperiod during which the surplus heat accumulated in the insulatingmaterial 7 is used because the surplus heat accumulated in theinsulating material is used after a correction to the basic fuel supplytable begins, and the stored surplus heat gradually decreases as timepasses during which a correction is being executed, and therefore theamount of correction to the fuel utilization rate declines when thestored surplus heat declines.

Note that it is also acceptable not to make a correction using thesecond modifying coefficient.

Next, in step S7, the rate change determined in step S4 is multiplied bythe first modifying coefficient determined in step S5 and the secondmodifying coefficient determined in step S6 to determine a finalutilization rate change. Moreover, the amount of correction to the watersupply rate is determined according to the determined fuel supply rate,and the generating air supply rate is reduced by 10% from the normal airsupply rate. Also, the fuel supply rate control gain is increased by 10%from the control gain used for the normal operation, thereby improvingthe responsiveness of changing the fuel supply rate.

Thus, by increasing the fuel supply rate control gain while correctionsare being made to the basic fuel supply table to thereby increase theresponsiveness of changing the fuel supply rate, the fuel supply ratecan be quickly increased while the fuel utilization rate is beingmodified to decline (fuel supply rate is increased) with a reduction ofthe estimated amount of stored surplus heat. The fuel cell module 2 isprevented from being cooled down by a time delay placed before the fuelsupply rate is increased. Therefore, the control to increase the gain instep S7 acts as an excess cooling prevention circuit. Since a reductionof the secondary generating air rate by 10% suppresses the cells,reformer, etc., inside fuel cell module 2 from being cooled down, areduction of the stored surplus heat amount is also suppressed, and thestored surplus heat can be effectively utilized. As a result, thecontrol to reduce the secondary air rate by 10% also acts as an excesscooling prevention circuit.

In step S8, the control section 110 sends a signal to the fuel flowregulator unit 38, the water flow regulator unit 28, and the oxidant gassupply device 45, and the amounts of fuel, water, and generating airdetermined in step S7 are supplied to the fuel cell module 2. After stepS8, one iteration of the process in the flow chart of FIG. 11 iscompleted. When the integrated value Ni declines to 0 as a result ofexecuting corrections to the basic fuel supply table, the processproceeds from step S2 to step S3. A correction to the basic fuel supplytable thus ends, and control of the fuel supply rate defined in thebasic fuel supply table is again executed.

Next, referring to FIGS. 12 and 13, we discuss estimation of the surplusheat accumulated in the insulating material 7.

Estimation of stored surplus heat is executed by a stored heat amountestimating circuit 110 b (FIG. 6) built into the control section 110.When step S1 in the flow chart shown in FIG. 11 is executed, the storedheat amount estimating circuit 110 b reads the temperature of thegenerating chamber from the generating chamber temperature sensor 142serving as temperature detection device. Next, the stored heat amountestimating circuit 110 b refers to the stored heat estimating tableshown in FIG. 12 and determines an add/subtract value based on thetemperature Td detected by the generating chamber temperature sensor142. For example, when the detected temperature Td is 645° C., 1/50,000is determined as an addition value, and this value is added to theintegrated value Ni. An addition of this type is executed at apredetermined time after the startup of the solid oxide fuel cell 1. Inthis embodiment, the flow chart in FIG. 11 is executed every 0.5seconds. Therefore, an addition is executed every 0.5 seconds.Therefore, when the detected temperature Td is fixed at 645° C., forexample, a value of 1/50,000 is added every 0.5 seconds, and theintegrated value Ni grows.

This integrated value Ni reflects a history of temperature changes takenplace in the fuel cell module 2, and the generating chamber and servesas an index indicative of the amount of surplus heat accumulated in theinsulating material 7. The integrated value Ni is limited to a range of0 to 1. When the integrated value Ni reaches 1, the value is held at 1until the next subtraction occurs. When the integrated value Ni hasdeclined to 0, the value is held at 0 until the next addition takesplace. In the present invention, it is assumed that the value serving asan index indicative of the amount of stored surplus heat is an estimatedamount of the stored surplus heat. Therefore, in the present invention,the amount of stored surplus heat is estimated based on the temperatureof the fuel cell module 2.

An amount of utilization rate change made to the basic fuel supplytable, which is calculated in step S4 of the flow chart shown in FIG.11, is derived by multiplying a predetermined correction value to theintegrated value Ni. Therefore, the larger the integrated value Nibecomes, which serves as an estimated amount of stored surplus heat, themore the utilization rate increases. The utilization rate change becomesa maximum when the integrated value Ni is 1, and when the integratedvalue Ni is 0, no correction is executed (utilization rate change=0).For example, when the integrated value Ni is 0, the estimated amount ofstored surplus heat is judged to be under the amount of stored surplusheat sufficient for executing corrections to the basic fuel supplytable, and no correction to the fuel utilization rate is executed.

As shown in FIGS. 12 and 13, in the present embodiment, the addingoperation is carried out as an addition when the detected temperature Tdis higher than the reference temperature Tcr of 635° C., and as asubtraction when it is lower than same. That is, the integrated value Niis calculated assuming that when the detected temperature Td is higherthan the reference temperature Tcr, surplus heat usable for increasingthe fuel utilization rate is accumulated in the insulating material 7,and when lower than the change reference temperature Tcr, the surplusheat accumulated in the insulating material 7 decreases. Put anotherway, the integrated value Ni corresponds to integration over time of thetemperature deviations of the detected temperature Td from the referencetemperature Tcr, and the amount of stored surplus heat is estimatedbased on the integrated value Ni.

Note that in this embodiment, the reference temperature Tcr, whichserves as a reference temperature for estimating the amount of storedsurplus heat is set slightly higher than the heat storage temperatureTh, at which accumulation of surplus heat begins (FIG. 10). For thisreason, the amount of the stored surplus heat is estimated slightly lessthan the actual. Therefore, the fuel cell module 2 is avoided from beingcooled down by execution of excessive corrections to raise the fuelutilization rate based on the amount of stored surplus heat estimatedhigher than the actual.

Therefore, a correction is made to the basic fuel supply table when thegenerated power has declined while the detected temperature Td is higherthan the reference temperature Tcr. On the other hand, when thegenerated power has declined while the detected temperature Td is lowerthan the reference temperature Tcr, the amount of correction to thebasic fuel supply table is reduced (by a decline of the integrated valueNi), or no correction is made (when the integrated value Ni is 0).

Specifically, as shown in FIGS. 12 and 13, when the detected temperatureTd is below 580° C., 20/50,000 is subtracted from the integrated valueNi. When the detected temperature Td is 580° C. or above and less than620° C., 10/50,000×(620−Td)/(620−580) is subtracted from the integratedvalue Ni. When the detected temperature Td is 620° C. or above and lessthan 630° C., 1/50,000 is subtracted from the integrated value Ni. Thus,the more rapidly the integrated value Ni decreases the lower thedetected temperature Td is than the reference temperature Tcr, and themore rapidly the amount of correction to the fuel utilization ratedecreases.

On the other hand, when the detected temperature Td is 650° C. or above,1/50,000×(Td−650) is added to the integrated value Ni. When the detectedtemperature Td is 640° C. or above and less than 650° C., 1/50,000 isadded to the integrated value Ni. Thus, the more rapidly the integratedvalue Ni increases the higher the detected temperature Td is than thereference temperature Tcr, and with this the amount of correction to thefuel utilization rate decreases.

Furthermore, in the range where the detected temperature Td is between630° C. and 640° C., different processes are performed depending onwhether the detected temperature Td is increasing or decreasing.

For example, when the detected temperature Td is between 630° C. and632° C., an addition value is 0 (no add/subtract is performed) when thedetected temperature Td is rising, whereas 1/50,000 is subtracted whenit is falling. Thus, when the detected temperature Td is less than thereference temperature Tcr, and the difference between them is small or5° C. or below, the integrated value Ni decreases more rapidly when thedetected temperature Td is falling than when it is increasing. Here,when the insulating material 7 has an extremely high heat capacity, andthe detected temperature Td has started decreasing, it is expected thatthe temperature will continue to drop for a certain period of time.Therefore, in such circumstances, it is necessary to avoid the risk ofsuffering a major temperature drop in the fuel cell module 2 by quicklyreducing the integrated value Ni and suppressing corrections to lowerthe fuel utilization rate (reducing the fuel supply rate).

On the other hand, when the detected temperature Td is between 638° C.and 640° C., 1/50,000 is added when the detected temperature Td isincreasing, whereas 0 is added (no add/subtract is performed) when it isdeclining. As described above, when the insulating material 7 has anextremely high heat capacity, and the detected temperature Td hasstarted increasing, it is expected that the temperature will continue torise for a certain period of time. Therefore, in such circumstances thestored surplus heat is actively utilized to improve the fuel utilizationrate by promoting corrections to raise the fuel utilization rate (reducethe fuel supply rate) by quickly increasing the integrated value Ni.

Different values are added to or subtracted from the integrated value Niaccording to how the detected temperature Td is changing. Therefore, therelationship between a temperature deviation between the detectedtemperature Td and the reference temperature Tcr, and the integratedvalue Ni reflecting the amount of stored surplus heat is changedaccording to how the detected temperature Td is changing.

Also, when the detected temperature Td is 632° C. or above and less than638° C., and the detected temperature Td is close to the referencetemperature Tcr of 635° C. and is deemed to be stable, 0 is addedregardless of how the detected temperature Td is changing, and thecurrent status is maintained.

Next, referring to FIG. 16, we explain the processes performed when thefuel cell module 2 has degraded. FIG. 16 is a flowchart of the processesfor changing correction amounts when the fuel cell module 2 hasdegraded.

When degradation of the fuel cell units 16 advances after long years ofuse, the power extractable at a given fuel supply rate declines. Inconjunction with this, the temperature of the fuel cell units 16 alsorises for the same power produced. In the solid oxide fuel cell 1 of thepresent embodiment, a determination of degradation of the fuel cellmodule 2 (the fuel cell units 16) is made based on the temperature ofthe fuel cell module 2 measured when a predetermined electrical power isgenerated. Note that degradation of a fuel cell module can also bedetermined from the power or voltage, etc. which can be extracted at apredetermined fuel supply rate.

The flow chart shown in FIG. 16 is executed at a predetermined intervalwhich is, for example, several months to several years by the controlsection 110. First, in step S21 of FIG. 16, a judgment is made as towhether the fuel cell units 16 have degraded. If it is determined thatthe fuel cell units 16 have not degraded, the process of one iterationof the flow chart shown in FIG. 16 ends. If it is determined that thefuel cell units 16 have degraded, the process advances to step S22.

In step S22, the reference temperature Tcr is changed by 5° C. higher,and a third modifying coefficient is set to 0.8. Then, the process ofone iteration of the flowchart shown in FIG. 16 ends. This change ismade because when the fuel cell units 16 have degraded, the operatingtemperature of the fuel cell module 2 generally shifts higher and thetemperature used to correct the reference. The third modifyingcoefficient is multiplied by the utilization rate change determined instep S4 of FIG. 11. The third modifying coefficient is set to 1 prior tothe degradation of the fuel cell units 16, and when it is determinedthat degradation has occurred, it is changed to 0.8, and the utilizationrate change is reduced by 20%. The fuel cell units 16 is prevented frombeing further degraded by large corrections to the fuel utilization rateafter the fuel cell units 16 has degraded. Note that after it isdetermined that the fuel cell module 2 has degraded, the thresholdtemperature used to determine degradation is changed for a next timefurther degradation is determined. It is therefore possible to determinethe progress of degradation over time at a number of times. The value ofthe reference temperature Tcr is also changed each time an occurrence ofdegradation is determined.

Next, referring to the FIG. 17, we discuss the operation of a solidoxide fuel cell 1 according to the first embodiment of the presentinvention. FIG. 17(a) is a diagram conceptually showing the operation ofa solid oxide fuel cell 1 according to the present embodiment, and FIG.17(b) schematically shows changes in power demand over a day in anexemplary household. The upper graph in FIG. 17(a) conceptuallyillustrates the operation when no amount of surplus heat is accumulatedin the insulating material 7. The middle and lower graphs show cases,respectively, where accumulated surplus heat is small and large. When,as shown in the upper portion of FIG. 17(a), the time duration ofoperation to increase the fuel supply rate is short, the amount ofsurplus heat is not accumulated in the insulating material 7. Therefore,the operation performed after the generated power has declined isperformed at fuel supply rates defined in the basic fuel supply table,and the fuel utilization rate will not increase. When, on the otherhand, a high power generation operation continues for a certain timeperiod as in the middle portion of FIG. 17(a), the operation performedafter the generated power has declined is carried out by utilizing theamount of surplus heat accumulated in the insulating material 7 when thepower generation was large. Therefore, a high-efficiency operation withthe fuel supply rate reduced from that of the basic fuel supply table iscarried out during the period that a utilizable amount of surplus heatremains in the insulating material 7. Thus, the amount of fuelcorresponding to the shaded area of the middle graph is saved.Furthermore, when a large generated power operation is carried out for along period of time as shown in the bottom portion of FIG. 17(a), alarge amount of surplus heat is accumulated in the insulating material7. Therefore, a high-efficiency operation utilizing the accumulatedsurplus heat is carried out over a longer time, and an even more amountof fuel is saved.

Next, in FIG. 17(b), a change of the power demand in an exemplaryhousehold is shown by a solid line. The power generated by the solidoxide fuel cell 1 is shown by a dotted line, and the integrated value Niserving as an index of the amount of stored surplus heat is shown by adot and dash line.

First, at time t0-t1 when household members are asleep, the power demandin the household is small. At time t1, the occupants awake and the powerdemand increases. In conjunction with this, the power generated from thesolid oxide fuel cell 1 also increases, and that portion of the demandedpower exceeding the rated power of the fuel cell is supplied from thepower grid. Since a low power demand state in which power use is smallcontinues for approximately 6 to 8 hours during which the occupants areasleep, the accumulated of surplus heat (integrated value Ni) estimatedby the stored heat amount estimating circuit 110 b is 0 or an extremelysmall value.

When, at time t1, the generated power increases, and the fuel cellmodule 2 is operating at a temperature higher than the heat storagetemperature Th, the amount of stored of surplus heat graduallyincreases, and at time t2, the integrated value increases toapproximately 1, which is the maximum integrated value. Thereafter, thepower demand suddenly drops when the occupants leave the house at timet3. Thus, when the generated power drops in a state that the amount ofstored surplus heat is equal to or greater than the amount sufficient tobe able to change the fuel utilization rate, a correction to the basicfuel supply table by the fuel table change circuit 110 a is executed,and the fuel utilization rate is increased while the generated power islow. During the operation at a raised the fuel utilization rate, thesurplus heat amount accumulated in the insulating material 7 isutilized, and the integrated value Ni also declines. In the embodiment,the operation at an improved fuel utilization rate possible forapproximately 1 to 3 hours.

Next, when the occupants return home at time t4, the power demand againincreases. The integrated value Ni increases with some delay (timet4-t5) after the increase of the power demand at time t4 and againreaches the maximum value. Next, at time t6 the occupants go to bed. Theoperation at an increased fuel utilization rate starts after the powerdemand has decreased (time t6 and later).

When the power demand at a household changes in this manner, theoperation at an increased fuel utilization rate, in which the surplusheat amount stored in the insulating material 7 is utilized, is carriedout twice a day. The operations at a high fuel utilization rate last foras much as 20-50% of the period when the generated power is small, andimprove the overall energy efficiency of the solid oxide fuel cell 1.

In conventional solid oxide fuel cells, when the generated power issmall, heat generated by generating power drops, and for the temperatureof the fuel cell module is prone to drop. In order to prevent anexcessive temperature drop, the fuel utilization rate is thereforereduced when the generated power is low to burn an amount of fuel notused for electrical generation to heat up the fuel cell module. Inparticular, in solid oxide fuel cells of a type in which the reformer isdisposed within the fuel cell module, endothermic reactions occur insidethe reformer, which increase the chance of temperature drops.

In the solid oxide fuel cell 1 of the present invention, when thegenerated power is small, if it is estimated by the stored heat amountestimating circuit 110 b that a usable amount of surplus heat hasaccumulated in the insulating material 7, the basic fuel supply table istemporarily corrected so that the fuel utilization rate increases (FIG.11, step S7). Thermal independence of the solid oxide fuel cell 1 isthus maintained, and the overall energy efficiency of the solid oxidefuel cell 1 is improved, while excessive temperature drops are avoided.

The solid oxide fuel cell 1 of the present embodiment is formed (FIG.10) so that more surplus heat is estimated to be accumulated in theinsulating material 7 in a temperature range above a predeterminedstorage heat temperature Th. Therefore, the accumulated surplus heat canbe effectively utilized by actively accumulating surplus heat in aregion where the temperature is higher than the heat storage temperatureTh and the fuel utilization rate can be raised and by consuming theaccumulated surplus heat while the generated power is small, and thetemperature of the fuel cell module 2 is comparatively low, so that thestored surplus heat is easy to utilize.

In the solid oxide fuel cell 1 of the present embodiment, since thedetected temperature Td detected by the generating chamber temperaturesensor 142 generally reflects the amount of surplus heat stored in theinsulating material 7, the basic fuel supply table can be easilycorrected using the relationship between the detected temperature Td andthe reference temperature Tcr.

In the solid oxide fuel cell 1 of the present embodiment, since thereference temperature Tcr is set higher than the heat storagetemperature Th (FIG. 10), the surplus heat is estimated to be stored ator above the reference storage temperature Tcr, which is higher than theheat storage temperature Th, at which surplus heat begins tp be storedin the insulating material 7. As a result, the risk can be avoided thatthe stored surplus heat is used despite the fact that the stored amountof surplus heat, resulting in an excessive temperature drop.

In the solid oxide fuel cell 1 of the present embodiment, since thestored heat amount estimating circuit 110 b estimates the amount ofstored surplus heat based on the history of the detected temperature Td(FIG. 11, step S4, FIG. 13), a more accurate estimate can be madecompared to estimating the amount of stored surplus heat using thetemperature Td alone, and the stored surplus heat can be moreeffectively used.

In the solid oxide fuel cell 1 of the present embodiment, since theamount of surplus heat accumulated in the insulating material 7 isestimated by accumulating temperature deviations over time (FIG. 11,step S4, FIG. 13, when the time during which the operation is performedat a temperature higher than the stored heat temperature Th is long, theestimated amount of stored surplus heat is large, whereas when the timeis short, the estimated amount of stored surplus heat is small, and amore accurate estimation of the amount of stored surplus heat can beachieved. The risk of excessive temperature drops due to utilization ofthe stored surplus heat can thus be reliably avoided.

In the solid oxide fuel cell 1 of the present embodiment, since theamount of correction to raise the fuel utilization rate is increased asthe amount of stored surplus heat increases (FIG. 11, step S4, FIG. 13),a correction can be performed to greatly improve the fuel utilizationrate while the risk of excessive temperature drops can be reliablyavoided.

In the solid oxide fuel cell 1 of the present embodiment, since the morethe correction amount increases the higher the detected temperature Tdis, whereas the more the correction amount decreases the lower thedetected temperature Td is (FIG. 13), a large correction to the fuelutilization rate can be made when the detected temperature Td is high,and the correction amount can be swiftly decreased when the detectedtemperature Td is low, so that excessive temperature drops can bereliably prevented.

In the solid oxide fuel cell 1 of the present embodiment, since therelationship between the estimated amount of stored surplus heat and thecorrection amount is changed according to a rise or fall of the detectedtemperature Td or the generated power (FIG. 13, 630-640° C.; FIGS. 14,15, 18), the two goals of preventing excessive temperature drops andeffectively utilizing stored heat can be achieved.

In the solid oxide fuel cell 1 of the present embodiment, since the fueltable change circuit 110 a reduces the correction amount when thegenerated power is small (FIG. 15), the usable amount of stored surplusheat declines, and the period during which the stored surplus heat canbe used can be extended.

In the solid oxide fuel cell 1 of the present embodiment, since theestimated amount of stored surplus heat drops to a large degree when thedetected temperature Td is declining, and the difference between thedetected temperature Td and the reference temperature Tcr is at or belowa predetermined very small deviation value (FIG. 13, 630-632° C.), theestimated amount of the stored surplus heat is swiftly reduced when thedetected temperature Td is declining, and excessive temperature dropscan be reliably prevented.

In the solid oxide fuel cell 1 of the present embodiment, since thecorrection amount to increase the fuel utilization rate is changedaccording to the state of the fuel cell module 2 (FIGS. 14, 15, 16),corrections to the fuel utilization rate not conforming to the state ofthe fuel cell module 2 can be prevented.

In the solid oxide fuel cell 1 of the present embodiment, since thereference temperature Tcr is changed higher when the fuel cell module 2has degraded (FIG. 16), the fuel utilization rate can be correctedwithout placing an excessive burden on the fuel cell module 2 when ithas degraded and its operating temperature has risen.

In the solid oxide fuel cell 1 of the present embodiment, since thecorrection amount is reduced when the fuel cell module 2 has degraded(FIG. 16, step S22), the progress of degradation can be suppressed bycorrecting the fuel utilization rate.

Also, in the above-described first embodiment of the present invention,an add/subtract value made to the integrated value Ni is calculatedbased on only the detected temperature Td shown in the stored surplusheat estimation table shown in FIG. 12. However, as a variant example,an add/subtract value can also be determined depending on the outputcurrent. For example, the integrated value Ni can be calculated byintegrating values obtained by multiplying the add/subtract valuesdetermined based on the stored surplus heat estimation table (FIG. 12)to a current correction coefficient shown in FIG. 18. As shown in FIG.18, the current correction coefficient is determined as 1/7 for anoutput current of 3 A or below and 1/12 for 4 A or above, and islinearly decreased from 1/7 to 1/12 between 3 and 4 A.

By multiplying by the current correction coefficient set in this manner,the integrated value Ni drops swiftly in a range where the generatedpower is small. Increases and decreases of the integrated value Ni in arange where the generated power is medium or greater become gradual.Therefore, by correcting the basic fuel supply table, the integratedvalue Ni is gradually decreased while the generated power is small,during which a large amount of surplus heat accumulated in theinsulating material 7 is consumed. The risk of inducing extraordinarytemperature drops by overestimating the amount of stored surplus heatcan thus be reliably avoided.

In the above-described embodiment, a value for addition to orsubtraction from the integrated value Ni is determined by the detectedtemperature Td alone, as shown in FIG. 13. However, the presentinvention can also be configured so that the add/subtract value is alsodependent on the output current. For example, at an output current of 3A or below (an output power of 300 W), the reference temperature Tcr canbe raised by about 2° C., and the entire FIG. 13 graph is shifted byabout 2° C. In this manner, the reference temperature Tcr is changedhigher when the generated power is small, and the estimated amount ofstored surplus heat is calculated small. The correction amount toincrease the fuel utilization rate is thus reduced. Therefore, the fuelutilization rate is greatly improved in the range where the generatedpower is small and the fuel supply rate is low, so that excessive dropsin the fuel supply rate can be suppressed.

Next, referring to the FIGS. 19 through 33, we discuss a solid oxidefuel cell according to a second embodiment of the present invention.

In the solid oxide fuel cell of the present embodiment, control by thecontrol section 110 is different from that described above for the firstembodiment. Therefore, we explain only the portions of the secondembodiment of the invention which differ from the first embodiment, andwe omit explanation of similar constitutions, operations, and effects.

In the above-described first embodiment, the fuel supply rate wasdetermined based on the basic fuel supply table in response to a powerdemand, and the determined fuel supply rate is temporarily changed sothat the determined fuel supply rate is reduced based on the amount ofsurplus heat accumulated in the insulating material 7, therebytemporarily increasing the fuel utilization rate. Thus, in the solidoxide fuel cell of the second embodiment, no process is conducted todetermine the fuel supply rate based on the basic fuel supply table andchange the fuel supply rate based on an estimated amount of storedsurplus heat. Rather, the fuel supply rate is directly calculated basedon a detected temperature Td. In the present embodiment, however, thefuel supply rate, directly determined based on the detected temperatureTd, includes an addition of the amount of surplus heat accumulated inthe insulating material 7, and the fuel utilization rate is improved byutilizing the stored surplus heat when the amount of stored surplus heatis large. Therefore, the same technical concept as in the firstembodiment can be achieved.

Next, in the above-described first embodiment, the change made to thefuel supply rate to increase the fuel utilization rate based on theestimated amount of stored surplus heat is accomplished by multiplyingthe change amount to a first correction coefficient (FIG. 11 step S5,FIG. 14), so that it is primarily used when the generated power is small(at or above a generated power of 4.5 A; first correctioncoefficient=0). In contrast, in the present embodiment, a coefficientcorresponding to the first correction coefficient in the firstembodiment is not used. Therefore, in the present embodiment, highefficiency control utilizing the amount of surplus heat accumulated ininsulating material 7 is executed not only in the range where thegenerated power is low, but also in the range where the generated poweris high. Therefore, in the solid oxide fuel cell of the presentembodiment, in addition to the effect of improving the fuel utilizationrate using stored surplus heat, the effect of consuming the amount ofsurplus heat accumulated in the insulating material 7 to therebysuppress temperature rises is obtained when the temperature of the fuelcell module 2 rises excessively. Note that in the above-described firstembodiment, if the first modifying coefficient is omitted (the changeamount is not multiplied by the first modifying coefficient), the sameeffect can be obtained.

FIG. 19 is a graph schematically showing the relationship betweenchanges in power demand, fuel supply rate, and the current actuallyextracted from a fuel cell module 2. FIG. 20 is a graph showing anexample of the relationship between the generating air supply rate,water supply rate, fuel supply rate, and the current actually extractedfrom a fuel cell module 2.

As shown in FIG. 19, the fuel cell module 2 is controlled to producepower to meet the power demand shown in FIG. 19(i). Based on the powerdemand, the control section 110 sets the fuel supply current value If,which is the target current to be achieved by the fuel cell module 2, asshown in FIG. 19(ii). The fuel supply current value If is set to roughlyfollow changes in the power demand. But since the speed of the fuel cellmodule 2 responding to changes in the power demand is extremely slow, itis set to follow the power demand gradually and does not follow suddenchanges in the power demand which occur at short cycles. When the powerdemand exceeds the maximum rated power of the solid oxide fuel cell, thefuel supply current value If follows up to the current valuecorresponding to the maximum rated power, and does not get set tocurrent values above that.

The control section 110 controls the fuel flow regulator unit 38 servingas a fuel supply device in the manner shown in the FIG. 19(iii) graphand supply fuel to the fuel cell module 2 at a fuel supply rate Frsufficient to produce the power corresponding to the fuel supply currentvalue If. Note that if the fuel utilization rate is constant, whichrepresents a ratio of the fuel actually used to generate electricity tothe fuel supply amount, the fuel supply current value If and the fuelsupply current value Fr are proportional to each other. In FIG. 19, thefuel supply current value If and the fuel supply current value Fr aredrawn as being proportional to each other. But as described below, inactuality the fuel utilization rate is not fixed constant in thisembodiment.

Moreover, as shown in FIG. 19(iv), the control section 110 outputs asignal to the inverter 54 to output extractable current Iinv, which isthe current value which can be extracted from the fuel cell module 2.The inverter 54 extracts current (power) from the fuel cell module 2 inresponse to the power demand, which changes rapidly from a moment toanother within the range of the extractable current Iinv. A portion ofthe power demand exceeding the extractable current Iinv is supplied fromthe power grid. Here, as shown in FIG. 19, the extractable current Iinvinstructed to the inverter 54 by the control section 110 is set tochange with a predetermined time delay placed after the fuel supplycurrent value Fr is changed when the current is rising. For example, attime t10 in FIG. 19, the extractable current Iinv begins to increasewith a delay after the fuel supply current value If and the fuel supplycurrent value Fr start to rise. At time t12, as well, the extractablecurrent Iinv starts to increase with a delay after the fuel supplycurrent value If and the fuel supply current value Fr increase. Thus,the delay time placed before the power actually extracted from the fuelcell module 2 is increased after an increase in the fuel supply currentvalue Fr makes it possible to accommodate the delay time which fuelsupplied to the fuel cell module 2 takes to pass through the reformer20, etc. to reach the individual fuel cell stack 14, and the delay timewhich the electrical generation reaction takes to take place after fuelreaches the individual fuel cell stack 14, and so forth. Therefore, theoccurrence of fuel cut-off in each of the fuel cell units 16 and theresulting damage to the fuel cell units 16 can be reliably prevented.

FIG. 20 shows in more detail the relationship between changes of thegenerating air supply rate, water supply rate, and fuel supply rate andextractable current Iinv. Note that the graphs of the generating airsupply rate, water supply rate, and fuel supply rate shown in FIG. 20are each converted to the current values corresponding to those supplyrates. In other words, if the supplied generating air, water, and fuelare all set to the supply rates exactly necessary to generateelectricity, each of the supply rate graphs coincides with the graph ofthe extractable current Iinv. Therefore, the amount of differencebetween each of the supply rate graphs and the extractable current Iinvcorresponds to a surplus of each of the supply rates. The excess fuelnot used for generating electricity is burned in the combustion chamber18, which is the combustion section located above the individual fuelcell stack 14, and is used to heat the interior of the fuel cell module2.

As shown in FIG. 20, the generating air supply rate, water supply rate,and fuel supply rate are always above the extractable current Iinv.Current exceeding the current producible at the respective supply ratesis extracted from the fuel cell module 2, thereby preventing damage tothe fuel cell units 16 due to a fuel shortage, an air shortage, and thelike. For the fuel supply rate exceeding the extractable current Iinv,the water supply rate is set to the supply rate at which all of thesupplied fuel can be steam reformed. That is, in order for the suppliedfuel to be all steam-reformed, the water supply rate is set to satisfythe ratio S/C between the amount of steam needed for steam reforming andthe amount of carbon contained in the fuel. Carbon deposition inside thereformer is thus prevented. In regions A and C of FIG. 20, in which theextractable current Iinv is increasing with an increase of the powerdemand, an offset of the fuel supply rate, etc. is set higher than inthe B region, where the extractable current Iinv is flat. When thegenerated power is increased, the generated power takes a delay time toincrease after the fuel supply rate at which fuel is supplied to thefuel cell module 2 is increased by a power extraction delay circuit 110c (FIG. 6) built into the control section 110. That is, the poweractually output from the fuel cell module 2 is changed with a delay timeplaced after the fuel supply rate is changed in response to changes ofthe power demand. In addition, when the extractable current Iinv israpidly reduced in response to a drop of the power demand (region C,beginning of region D), each supply rate is reduced with a predetermineddelay time placed after the reduction of the extractable current Iinv.Therefore, an extremely large amount of excess fuel is supplied after arapid reduction of the extractable current Iinv. Thus, in cases wherethe power demand suddenly drops, a sudden reduction of the extractablecurrent Iinv is implemented to prevent a reverse current flow. Thus,when the generated power increases and when the generated powerdecreases, more excess fuel is produced than when the generated power isconstant, and the excess fuel is used to heat the fuel cell module 2.Therefore, not only in cases where the fuel cell module 2 is operated tooutput high generated power for long hours, but also in cases where thegenerated power is frequently increased and decreased, the fuel cellmodule 2 is highly heated, and a large amount of surplus heat isaccumulated in the insulating material 7.

The solid oxide fuel cell of the present embodiment utilizes surplusheat in circumstances when the generated power has declined which isaccumulated not only while it is operated to output high generated powerfor a long time but also while the generated power is increases ordecreased.

Next, referring to FIGS. 21 through 28, we discuss a procedure fordetermining the generating air supply rate, water supply rate, and fuelsupply rate based on the detected temperature Td.

FIG. 21 is a flowchart showing the order in which generating air supplyrate, water supply rate, and fuel supply rate are determined based onthe detected temperature Td. FIG. 22 is a graph showing the temperatureof the fuel cell stack 14 temperature appropriate to the generatedcurrent. FIG. 23 is a graph showing the fuel utilization rate determinedaccording to an integrated value. FIG. 24 is a graph showing a range ofthe fuel utilization rates determined with respect to the generatedcurrent. FIG. 25 is a graph showing the air utilization rate determinedaccording to the integrated value. FIG. 26 is a graph showing a range ofthe air utilization rates determined with respect to the generatedcurrent. FIG. 27 is a graph for determining the water supply rate withrespect to the determined air supply utilization rate. FIG. 28 is agraph showing the voltage generated by the fuel cell module 2 which isappropriate with respect to the generated current.

As shown by the dot-and-dash line in FIG. 22, in this embodiment anappropriate temperature Ts(I) for the individual fuel cell stack 14 isdefined relative to the current being produced by the fuel cell module2. The control section 110 controls the fuel supply rate, etc. so thatthe temperature of the individual fuel cell stack 14 approaches theappropriate temperature Ts(I). Generally speaking, that is, when thetemperature of the individual fuel cell stack 14 is high relative to thegenerated current (when the individual fuel cell stack 14 temperature isabove the dot-and-dash line in FIG. 22), the fuel utilization rate isincreased, and the surplus heat amount accumulated in the insulatingmaterial 7, etc. is actively consumed, and the temperature inside thefuel cell module 2 is thereby reduced. Conversely, when the temperatureof the individual fuel cell stack 14 is low relative to the generatedcurrent, the fuel utilization rate is reduced, and the temperatureinside the fuel cell module 2 is prevented from dropping. Specifically,the fuel utilization rate is not determined simply from the detectedtemperature Td alone. The fuel utilization rate is determined bycalculating an amount reflecting stored surplus heat which is derived byintegrating an add/subtract value determined based on detectedtemperature Td, etc. An estimation value for the amount of storedsurplus heat from the integration of add/subtract values is calculatedby a stored heat estimation circuit 101 b incorporated in the controlsection.

The flow chart shown in FIG. 21 determines the generating air supplyrate, water supply rate, and fuel supply rate based on the detectedtemperature Td detected by the generating chamber temperature sensor 142serving as temperature detection device, and is executed at apredetermined time interval.

First, in step S31 of FIG. 21, a first add/subtract value M1 iscalculated based on the detected temperature Td and on FIG. 22. If thedetected temperature Td is in a predetermined temperature range from theappropriate temperature Ts(I) (between the two solid lines in FIG. 22),the first add/subtract value M1 is set to 0. That is, when the detectedtemperature Td is within the range ofTs(I)−Te≦Td≦Ts(I)+Te,the first add/subtract value M1 is set to 0. Here Te is the firstadd/subtract threshold temperature. Note that in the present embodiment,the first add/subtract threshold temperature Te is 3° C.

When the detected temperature Td is below the appropriate temperatureTs(I), i.e.,Td<Ts(I)−Te  (4)(below the bottom solid line in FIG. 22), the first add/subtract valueM1 is calculated by:M1=Ki×(Td−(Ts(I)−Te))  (5)In this calculation, the first add/subtract value M1 takes a negativevalue (a subtracting value). Note that Ki is a predeterminedproportional constant.

When the detected temperature Td is above the appropriate temperatureTs(I), i.e.,Td>Ts(I)+Te  (6)(above the bottom solid line in FIG. 22), the first add/subtract valueM1 is calculated by:M1=Ki×(Td−(Ts(I)+Te))  (7)In this calculation, the first add/subtract value M1 takes a positivevalue (an additive value). Thus, the first add/subtract value isdetermined based on the generated current in addition to the detectedtemperature Td, and the amount of stored surplus heat is estimated byintegrating these values. The appropriate temperature Ts(I), in otherwords, differs according to the generated current (power), and the firstadd/subtract value M1 is determined positive or negative based on thevalue of (Ts(I)+Te) or the value of (Ts(I)−Te) determined based on thisappropriate temperature Ts(I).

Note that when the detected temperature Td exceeds (Ts(I)+Te), the firstadd/subtract value M1 becomes a positive value, and the fuel supply rateis changed to raise the fuel utilization rate as described below.Therefore, in the present supplication, the temperature (Ts(I)+Te)relative to the generated power is referred to as a fuel utilizationrate change temperature. After high efficiency control begins in whichthe temperature exceeds the fuel utilization rate change temperature(Ts(I)+Te) and the fuel utilization rate is increased, the timing atwhich the operation goes back from the high efficiency control to targettemperature range control in which the amount of accumulated surplusheat is not consumed is, as described below, a point at which the firstintegrated values N1 id, such as the first add/subtract value M1 or thelike, decrease to 0. Therefore, even after the detected temperature Tdhas dropped below the fuel utilization rate change temperature(Ts(I)+Te), the first integrated value N1 id is maintained for a shorttime at a value greater than 0, and the high efficiency control isimplemented. Therefore, the temperature at which the operation goes backfrom the high efficiency control to the target temperature range controlis lower than the fuel utilization rate change temperature.

Next, in step S32 of FIG. 21, a second add/subtract value M2 iscalculated based on the latest detected temperature Td and the detectedtemperature Tdb detected one minute earlier. First, when the absolutevalue of the difference between the latest detected temperature Td andthe detected temperature Tdb detected one minute prior is less than asecond add/subtract value threshold value temperature, the secondadd/subtract value M2 is set to 0. Note that in the present embodiment,the second add/subtract threshold temperature is 1° C.

When a change temperature difference, which is the difference betweenthe latest detected temperature Td and the detected temperature Tdbdetected one minute prior, is equal to or greater than the secondadd/subtract value threshold value temperature, the second add/subtractvalue M2 is calculated byM2=Kd×(Td−Tdb)  (8)This second add/subtract value M2 is a positive value (additive value)when the detected temperature Td is rising and takes a negative value(subtractive value) when the detected temperature Td is falling. Notethat Kd is a predetermined proportional constant. Therefore in caseswhere the detected temperature Td is rising, in the range where thechange temperature difference (Td−Tdb) is large the second add/subtractvalue M2, which is a quick response estimate value, is increased morethan in the range where the change temperature difference is small.Conversely, in cases where the detected temperature is falling in therange where the absolute value of the change temperature difference(Td-Tdb) is large, the second add/subtract value M2 is decreased morethan in the range where the absolute value of the change temperaturedifference is small.

Note that in the present embodiment, the proportional constant Kd is afixed value, but as a variant example, a different proportional constantKd can be used in the case where the change temperature difference ispositive and the case where it is negative. For example, theproportional constant Kd can also be set high when the changetemperature difference is negative. Thus the quick response estimatevalue is changed relative to the change temperature difference morerapidly when the detected temperature is falling than when the detectedtemperature is rising. As a variant example, the proportional constantKd can also be set higher in the range where the absolute value of thechange temperature difference is high than in the range where it is low.This results in that the quick response estimate value is changedrelative to the change temperature difference more rapidly in the rangewhere the absolute value of the change temperature difference is highthan in the range where the absolute value of the change temperaturedifference is low. It is also possible to combine a change of theproportional constant Kd based on whether the change temperaturedifference is positive or negative and a change of the proportionalconstant Kd based on the size of the absolute value of the changetemperature difference.

Next, in step S33 of FIG. 21, the first add/subtract value M1 calculatedin step S31 and the second add/subtract value M2 calculated in step S32are added to the first integrated value N1 id. In the first integratedvalue N1 id, the usable amount of surplus heat accumulated in theinsulating material 7 and the like is represented by the firstadd/subtract value M1, and a recent change of the detected temperatureTd is represented by the second add/subtract value M2. In other words,the first integrated value N1 id can be used as an estimated amount ofsurplus heat amount accumulated in insulating material 7 and the like.The calculation is performed in a continuous manner each time the FIG.21 flow chart is executed after the operation of the solid oxide fuelcell system starts. The first add/subtract value M1 and the secondadd/subtract value M2 are added to or subtracted from a previouslycalculated first integrated value N1 id, and the first integrated valueN1 id is updated to a new value. The first integrated value N1 id islimited to a range between 0 and 4. When the first integrated value N1id reaches 4, the value is held at 4 until a next subtraction occurs.When the first integrated value N1 id has declined to 0, the value isheld at 0 until a next addition takes place.

Note that in step S33, the value of a second integrated value N2 id isalso calculated, in addition to the first integrated value N1 id. Asdescribed below, the second integrated value N2 id is calculated inexactly the same way as the first integrated value N1 id is calculateduntil degradation occurs in the fuel cell module 2, and takes the samevalue as the first integrated value N1 id takes.

Note that in this embodiment, as described above, an integrated value iscalculated by adding a sum of the first add/subtract value M1 and thesecond add/subtract value M2 to the first integrated value N1 id. Thatis, the first integrated value N1 id is calculated byN1id=N1id+M1+M2  (9)As a variant example, an integrated value can also be calculated byadding a product of the first add/subtract value M1 and the secondadd/subtract value M2. That is, in this variant example, the firstintegrated value N1 id is calculated byN1id=N1id+Km×M1×M2  (10)Here Km is a variable coefficient which is changed according topredetermined conditions. In this variant example, when the absolutevalue of the difference between the latest detected temperature Td andthe detected temperature Tdb detected one minute prior is less than thesecond add/subtract value threshold value temperature, the secondadd/subtract value M2 is set to 1.

Furthermore, in step S34 of FIG. 21, the fuel utilization rate isdetermined using the graphs in FIGS. 23 and 24, based on the calculatedfirst integrated value N1 id.

FIG. 23 is a graph showing the value for the fuel utilization rate Ufwith respect to the calculated first integrated value N1 id. As shown byFIG. 23, when the first integrated value N1 id is 0, the fuelutilization rate Uf is set to a minimum fuel utilization rate Ufmin. Thefuel utilization rate Uf increases with an increase of the firstintegrated value N1 id, and when the first integrated value N1 id=1,becomes a maximum fuel utilization rate Ufmax. Within the range, theslope of the fuel utilization rate Uf is small where the firstintegrated value N1 id is small, and the slope becomes steeper as thefirst integrated value N1 id approaches 1. In other words, the fuelutilization rate Uf is changed with a change of the amount of storedsurplus heat and is changed more where the estimated amount of storedsurplus heat is large than where the stored heat amount is small. Thatis, the fuel supply rate is reduced so as to greatly increase the fuelutilization rate Uf as the estimated stored heat amount increases.Furthermore, when the first integrated value N1 id is greater than 1,the fuel utilization rate Uf is fixed at the maximum fuel utilizationrate Ufmax. The specific values of these minimum fuel utilization rateUfmin and maximum fuel utilization rate Ufmax are determined using thegraph shown in FIG. 24, based on the generated current. Thus, when it isestimated that a usable amount of surplus heat is accumulated in theheat storing material 7 or the like, the fuel supply rate is reduced sothat the fuel utilization rate for a generated power is higher than itis for the same generated power when a usable amount of surplus heat isnot accumulated.

FIG. 24 is a graph showing a range of the fuel utilization rate Ufobtainable with respect to the generated current. The maximum andminimum values for the fuel utilization rate Uf are shown for thegenerated current. As shown in FIG. 24, the minimum fuel utilizationrate Ufmin for the generated current is set to increase as the generatedcurrent increases. That is, the fuel utilization rate is set high whenthe generated power is large, and the fuel utilization rate is set lowwhen the generated power is small. The straight line representing theminimum fuel utilization rate Ufmin corresponds to the basic fuel supplytable in FIG. 9 of the first embodiment. When the fuel utilization rateis sent to a value on this straight line, the fuel cell module 2 can bethermally autonomous without utilizing the amount of surplus heataccumulated in the insulating material 7 or the like.

The maximum fuel utilization rate Ufmax, on the other hand, is set tochange in a bent line fashion with respect to the generated current.Here the range of values which the fuel utilization rate Uf can takeswith respect to the generated current (the difference between themaximum fuel utilization rate Ufmax and the minimum fuel utilizationrate Ufmin) is narrowest when the generated current is maximum, andbroadens as the generated current declines. This is because in thevicinity of where the generated current is maximum, the minimum fuelutilization rate Ufmin is high at which thermal independence ispossible, and there is little margin for the fuel utilization rate Uf toincrease (decreasing the fuel supply rate) even if the stored surplusheat is used. Moreover, because the minimum fuel utilization rate Ufminat which thermal independence is possible declines as the generatedcurrent declines, the margin for the fuel supply rate to be reduced byutilizing stored surplus heat increases, and when there is a largeamount of stored surplus heat, the fuel utilization rate Uf can begreatly increased. Therefore, the fuel utilization rate is changed overa larger range where the generated power is small than where thegenerated power is large.

In the region below, a predetermined utilization rate suppressinggeneration amount IU at which the generated current is extremely small,the range of values the fuel utilization rate Uf can take is set smalleras the generated power decreases. This means that in the range wheregenerating current is small, the minimum fuel utilization rate Ufmin islow at which thermal independence is possible, and there is a margin forimprovement thereof. However, in the range where the generated currentis small, the temperature inside the fuel cell module 2 is low.Therefore, in this condition, if the operation is performed to greatlyimprove the fuel utilization rate Uf and rapidly consume the storedsurplus heat accumulated in the insulating material 7 or the like, thereis a risk of causing an excessive temperature drop inside the fuel cellmodule 2. Therefore, in the range where it is below a predeterminedutilization rate suppressing generation amount IU at which the generatedcurrent is extremely small, a change amount to increase the fuelutilization rate Uf is reduced as the generated power declines. In otherwords, the amount of change causing a reduction of the fuel supply rateas the amount of generation by the fuel cell module 2 declines. The riskof causing a sudden temperature drop can thus be avoided, and theaccumulated surplus heat can be utilized over a long time period.

In the present embodiment, the fuel supply rate is reduced by the fueltable change circuit 110 a built into the control section 110 so thatthe fuel utilization rate Uf increases relative to the minimum fuelutilization rate Ufmin. The fuel table change circuit 110 a does notchange the basic fuel supply table, but acts to raise the fuelutilization rate by changing the fuel supply rate which serves as thebase, thereby raising the fuel utilization rate, and thus the fuel tablechange circuit 110 a corresponds to the fuel table change circuit in thefirst embodiment.

In step S34 of FIG. 21, specific values of the minimum fuel utilizationrate Ufmin and the maximum fuel utilization rate Ufmax are determinedusing the graph in FIG. 24 based on the generated current. Next, thedetermined minimum fuel utilization rate Ufmin and maximum fuelutilization rate Ufmax are set according to the FIG. 23 graph, and thefuel utilization rate Uf is determined based on the first integratedvalue N1 id calculated in step S33.

Next, in step S35 of FIG. 21, an air utilization rate is determinedusing the FIGS. 25 and 26 graphs, based on the second integrated valueN2 id.

FIG. 25 is a graph showing values that the air utilization rate Ua cantake with respect to the calculated second integrated value N2 id. Asshown in FIG. 25, when the second integrated value N2 id is between 0and 1, the air utilization rate Ua is set to a maximum air utilizationrate Uamax, which is the maximum value. In addition, as the secondintegrated value N2 id exceeds 1 and increases, the air utilization rateUa declines, and when the second integrated value N2 id=4, it becomes aminimum air utilization rate Uamin, which is the minimum value. Thus,the increased amount of air caused by reducing the air utilization rateUa acts as a cooling fluid. Therefore, the air utilization rate Ua shownin FIG. 25 acts as a forced cooling circuit. The specific values ofthese minimum air utilization rate Uamin and the maximum air utilizationrate Uamax are determined using the graph shown in FIG. 26, based on thegenerated current.

FIG. 26 is a graph showing the range of values that the air utilizationrate Ua can take with respect to the generated current. The maximum andminimum values of the air utilization rate Ua are shown with respect tothe generated current. As shown in FIG. 26, the maximum air utilizationrate Uamax for the generated current is set to increase by a very smallamount as the generated current increases. On the other hand, theminimum air utilization rate Uamin decreases as generating currentincreases. Reducing the air utilization rate Ua (increasing the airsupply rate) more than the maximum air utilization rate Uamax results inthe introduction of a larger amount of air into the fuel cell module 2than is required for generation, causing the temperature inside the fuelcell module 2 to decline. Therefore, the air utilization rate Ua isreduced when the temperature in the fuel cell module 2 risesexcessively, and it is necessary to reduce the temperature. In thepresent embodiment, reducing the minimum air utilization rate Uamin(increasing the air supply rate) with a rise of the generated currentcauses the air supply rate corresponding to the minimum air utilizationrate Uamin to exceed the maximum air supply rate of the oxidant gassupply device 45 at a predetermined generated current. Therefore, in theregion in which the minimum air utilization rate Uamin is at or abovethe predetermined generated current shown by the dotted line in FIG. 26,it is not possible to achieve the air utilization rate Ua set by thegraph in FIG. 25. In such cases, the air supply rate is set to themaximum air supply rate of the oxidant gas supply device 45, regardlessof the set value of the air utilization rate Ua. In conjunction withthis, the air utilization rate Ua which is actually implementedincreases at or above a predetermined generated current. When theoxidant gas supply device having a large maximum air supply rate isused, the portion of the minimum air utilization rate Uamin shown by thebroken line in FIG. 26 can also be achieved. Note that the airutilization rate Ua defined by the maximum air supply rate of theoxidant gas supply device 45 is described as the limit minimum airutilization rate ULamin.

In step S35 of FIG. 21, the specific values of the minimum airutilization rate Uamin and the maximum air utilization rate Uamax aredetermined using the graph in FIG. 26, based on the generated current.Next, applying the determined minimum air utilization rate Uamin andmaximum air utilization rate Uamax to the FIG. 25 graph, the airutilization rate Ua is determined based on the second integrated valueN2 id calculated in step S33.

Next, in step S36 of FIG. 21, the ratio S/C of the steam amount to thecarbon amount is determined using FIG. 27, based on the air utilizationrate Ua determined in step S35.

FIG. 27 is a graph in which the horizontal axis shows the airutilization rate Ua and the vertical axis shows the ratio S/C of thesupplied steam amount to the carbon amount contained in the fuel.

First, in the range of the generated current in which the airutilization rate Ua set in step S35 is not defined by the maximum airsupply rate of the oxidant gas supply device 45 (between Uamax andULamin in FIG. 27), the value of the ratio S/C of the steam amount tothe carbon amount is fixed at 2.5. Note that a ratio of the steam amountto the carbon amount S/C=1 means that the entire amount of carboncontained in the supplied fuel is chemically steam reformed by thesupplied water (steam), without excess or shortage. Therefore, a ratioof the steam amount to the carbon amount S/C=2.5 means that steam(water) is supplied 2.5 times as much the minimum steam amountchemically as needed to steam reform the fuel. In actuality, theoperation is performed with the steam amount whose ratio is S/C=1,carbon deposition occurs inside the reformer 20. So a steam amount whoseratio is S/C=approximately 2.5 is the appropriate amount for steamreforming the fuel.

Next, in the range of the generated current in which the air utilizationrate Ua set in step S35 is limited by the maximum air supply rate of theoxidant gas supply device 45, the ratio S/C of the steam amount to thecarbon amount is determined using the graph in FIG. 27. In FIG. 27, thehorizontal axis is the air utilization rate Ua. The air supply ratedeclines as the air utilization rate Ua increases and approaches themaximum air utilization rate Uamax. When the air utilization rate Ua isreduced, on the other hand, and approaches the minimum air utilizationrate Uamin (the dotted line in FIG. 26), the air supply rate reaches thelimit, and the air utilization rate Ua becomes a limit minimum airutilization rate ULamin. As shown in FIG. 27, when the air utilizationrate Ua is larger (air supply rate is low) than the limit minimum airutilization rate ULamin, the ratio of the steam amount to the carbonamount is set at S/C=2.5. Additionally, when the air utilization rate Uadetermined in step S35 is smaller (air supply rate is large) than thelimit minimum air utilization rate ULamin (between Uamin and ULamin inFIG. 27), the ratio S/C of the steam amount to the carbon amountincreases as the air utilization rate Ua decreases, and at the minimumair utilization rate Uamin, it is set to S/C=3.5. That is, when the airutilization rate Ua determined in step S35 cannot be achieved using thelimit minimum air utilization rate ULamin (when the air utilization rateUa is determined to be within the region of the sloped line in FIG. 26),the ratio of the steam amount to the carbon amount S/C is increased, andthe water supply rate is increased. Thus, the temperature of thereformed fuel gas discharged from the reformer 20 is reduced, causingthe temperature inside the fuel cell module 2 to decline. Thus, when thewater supply rate is increased after the air utilization rate Ua isreduced and the air supply rate is increased, the increased amount ofwater (steam) acts as a cooling fluid, so that the water supply rateshown in FIG. 27 acts as a forced cooling circuit.

In step S37, the specific fuel supply rate, air supply rate, and watersupply rate are determined based on the fuel utilization rate Uf, theair utilization rate Ua, and the ratio S/C of the steam amount to thecarbon amount respectively determined in steps S34, S35, and S36. Inother words, the actual fuel supply rate is calculated by dividing thefuel supply rate—assuming the entire amount is used for electricalgeneration—by the determined fuel utilization rate Uf, and the actualair supply rate is calculated by dividing the air supply rate—assumingthe entire amount is used for electrical generation—by the determinedair utilization rate Ua. The water supply rate is calculated based onthe calculated fuel supply rate and the ratio S/C of the steam amount tothe carbon amount determined in step S36.

Next, in step S38, the control section 110 sends signals to the fuelflow regulator unit 38, the oxidant gas supply device 45, and the waterflow regulator unit 28 serving as water supply device, and supplies thefuel, air, and water at the rates calculated in step S37, therebycompleting the process of one iteration of the FIG. 21 flow chart.

Next, we discuss the time intervals at which the FIG. 21 flow chart isexecuted. In the present embodiment, the FIG. 21 flow chart is executedevery 0.5 seconds when the output current is large, and as outputcurrent falls, executed at twice as long as the interval, which is every1 second, 4 times as long as the interval, which is every 2 seconds, and8 times as long as the interval, which is every 4 seconds. Thus, whenthe first and second add/subtract values are fixed values, changes madeto the first and second add/subtract values at the interval become moregradual as the output current declines. That is, the stored heat amountestimating circuit 110 b changes the estimated amount of the storedsurplus heat amount at the interval more quickly as the output current(output power) increases. The estimated amount of stored surplus heatresulting from the integrated value thus accurately reflects the amountof actually stored surplus heat.

Next, referring to FIG. 28, we discuss the procedure for determining thefuel supply rate, air supply rate, and water supply rate when the fuelcell module 2 has degraded. FIG. 28 is a diagram showing the generatedvoltage relative to the generated current in the fuel cell module 2. Ingeneral, there is an internal resistance present in the individual fuelcell stack 14. Therefore, as shown in FIG. 28, the voltage drops whenthe current output from the fuel cell module 2 increases. Thedot-and-dash line shown in FIG. 28 shows the relationship between thegenerated current and the generated voltage when the fuel cell module 2has not degraded. In contrast, when the fuel cell module 2 has degraded,the internal resistance in the individual fuel cell stack 14 rises. As aresult, the generated voltage becomes smaller for the same generatedcurrent.

In the solid oxide fuel cell of the present embodiment, when thegenerated current drops by 10% or more relative to the initial generatedvoltage, and the generated voltage goes into the region below the solidline in FIG. 28, the fuel supply rate, air supply rate, and water supplyrate are determined by processing in a manner taking degradation intoconsideration.

For example, when the generated voltage is in the region below the solidline in FIG. 28, an integration of the first integrated value N1 id isstopped at step S33 in FIG. 21, and only integration of the secondintegrated value N2 id is continued. The value of the first integratedvalue N1 id used to determine the fuel utilization rate Uf withreference to the graph in FIG. 23 is fixed at a constant value. The fuelutilization rate Uf is therefore fixed until the generated voltage goesout of the region below the solid line in FIG. 28. Thus, changes toincrease the fuel utilization rate Uf are reduced more after the fuelcell module 2 has degraded than while the fuel cell module 2 has notdegraded. At the same time, the second integrated value N2 id used todetermine the air utilization rate Ua with reference to the graph inFIG. 26 is reduced as is reduced in the past, and increases anddecreases of the air utilization rate Ua are continued. The fuelutilization rate Uf is changed based on degradation of the fuel cellmodule 2, in addition to the first and second addition values reflectingthe estimated amount of stored surplus heat, and the power demand.

Next we discuss the operation of a solid oxide fuel cell using the flowchart shown in FIG. 21.

First, when the value of the first integrated value N1 id calculated instep S33 is 0, the fuel utilization rate Uf determined in step S34 isset at the minimum fuel utilization rate Ufmin (fuel supply ratemaximum) for the generated current. Thus, even in the state in which thefirst integrated value N1 id is 0, and the surplus heat accumulated inthe insulating material 7 or the like is small, sufficient fuel issupplied for the fuel cell module 2 to achieve thermal independence.When the value of the second integrated value N2 id calculated in stepS33 is 0, like the first integrated value N1 id, the air utilizationrate Ua determined in step S35 is set at the maximum air utilizationrate (air supply rate minimum) for that generated current. Therefore,cooling of the individual fuel cell stack 14 by the generating airintroduced into the fuel cell module 2 can be minimized, and thetemperature of the individual fuel cell stack 14 can be kept rising.

Next, when the fuel cell module 2 is operated in a state where thedetected temperature Td is higher than the appropriate temperatureTs(I), and Td>Ts(I)+Te, the value of the first add/subtract value M1becomes positive, and the value of first integrated value N1 id becomesgreater than 0. Thus, in FIG. 23, the fuel utilization rate Uf becomeshigher than the minimum fuel utilization rate Ufmin, and the fuel supplyrate is reduced, and the amount of excess fuel not used to generateelectricity is reduced. The fuel utilization rate Uf is greatlyincreased by the control section 110 as the value of the firstintegrated value N1 id reflecting the estimated amount of stored surplusheat increases. By increasing the fuel utilization rate Uf, the fuelsupply rate is reduced to below a supply rate at which thermalindependence is possible, and the high efficiency control is executed inwhich the amount of surplus heat accumulated in the insulating material7 and the like is used. The amount of excess fuel is reduced and theamount of surplus heat accumulated in the insulating material 7 or thelike is utilized. Therefore, the fuel table change circuit 110 asuppresses the rise of the temperature in the fuel cell module 2 whileelectricity continues to be generated. When operation is continued whenTd>Ts(I)+Te, an addition of the positive value to the first add/subtractvalue M1 is repeated, and the value of the first integrated value N1 idalso increases. When the first integrated value N1 id reaches 1, thefuel utilization rate Uf is set to the maximum fuel utilization rateUafmax (fuel supply rate minimum). Fuel supplied to the fuel cell module2 is determined based on the history of the detected temperature Td,which reflects the amount of surplus heat accumulated in the insulatingmaterial 7 or the like.

Even when the first integrated value N1 id further increases and exceeds1, the fuel utilization rate Uf is fixed at the maximum fuel utilizationrate Uafmax (fuel supply rate minimum), as shown in FIG. 23. On theother hand, when the second integrated value N2 id, which takes the samevalue as the first integrated value N1 id (when the fuel cell module 2is not degraded), also exceeds 1, the air utilization rate Ua declines(air supply rate increases) based on FIG. 25. Thus, the inside of thefuel cell module 2 is kept cooled because of the increase of thesupplied air.

By contrast, when the fuel cell module 2 is operated where the detectedtemperature Td is lower than the appropriate temperature Ts(I), andTd<Ts(I)−Te, the value of the first add/subtract value M1 becomesnegative, and the value of first integrated value N1 id is reduced. Thefuel utilization rate Uf is therefore maintained (first integrated valueN1 id>1) or decreased (first integrated value N1 id≦1). Also, the airutilization rate Ua increases (second integrated value N2 id>1) or ismaintained (second integrated value N2 id≦1). The temperature inside thefuel cell module 2 can thus be kept rising.

In the above-described operation of a solid oxide fuel cell, a focus isplaced only on the first add/subtract value M1 calculated based on thehistory of the detected temperature Td. However, the first integratedvalue N1 id and the second integrated value N2 id are also influenced bythe second add/subtract value M2. The fuel cell module 2, particularlythe individual fuel cell stack 14, has an extremely large heat capacity,and changes of the detected temperature Td thereof are extremely slow.Therefore once, the detected temperature Td starts rising, it isdifficult to suppress the temperature from rising in a short timeperiod, and when the detected temperature Td starts falling, a longperiod of time is required to bring it back to rising. Therefore, oncethe detected temperature Td starts rising or falling, an immediatecounteraction of modifying the first and second integrated values isrequired.

That is, when the latest detected temperature Td is above the detectedtemperature Tdb detected one minute prior by an amount equal to orgreater than the second add/subtract value threshold value temperature,the second add/subtract value M2 becomes a positive value, and the firstand second integrated values are increased. Therefore, the first andsecond integrated values reflect the behavior of the detectedtemperature Td, which is rising. Similarly, when the latest detectedtemperature Td is above the detected temperature Tdb detected one minuteprior by an amount equal or greater than the second add/subtract valuethreshold value temperature, the second add/subtract value M2 becomes anegative value, and the first and second integrated values aredecreased. In other words, the second add/subtract value M2, which is aquick response estimate value, is calculated by a change temperaturedifference, which is the difference between the latest detectedtemperature Td detected by the generating chamber temperature sensor 142and the detected temperature Tdb detected in the past. Therefore, whenthe detected temperature Td is steeply dropping, the change amountincreasing the fuel utilization rate Uf is more suppressed than when itis gradually dropping, and in the region where the generated power isbelow the suppressed utilization rate generation amount IU, since themaximum fuel utilization rate Ufmax is also set low, the change amountis greatly suppressed. Therefore, the first and second integrated valuesreflect the behavior of the detected temperature Td, which is falling.Thus, in the present embodiment, the amount of stored surplus heat isestimated based on an integrated value for the add/subtract determinedbased on detected temperature, and on the differential value between thenewly detected temperature and previously detected temperatures. Thatis, in the present embodiment, the amount of stored surplus heat isestimated by the stored heat amount estimating circuit 110 b based onthe integrated value of the first add/subtract value M1, which is abasic estimated value calculated based on the history of the detectedtemperatures Td, and based on the second add/subtract value M2, which isa quick response estimate value calculated based on the rate of changeof the detected temperature Td over a period of time shorter than thehistory of the basic estimated value calculation. Thus, in the presentembodiment, the amount of stored surplus heat is estimated based on thesum of the basic estimated value and the quick response estimate value.

Note that temperature changes in the fuel cell module 2 are extremelyslow compared to the 1 minute interval at which the detectedtemperatures Td and Tdb are detected Therefore, it is often the casethat the second add/subtract value M2 is 0. Therefore, the first andsecond integrated values are primarily dominated by the firstadd/subtract value M1, and the second add/subtract value M2 acts tomodify the values of the first and second integrated values when thedetected temperature Td starts rising or falling. Thus, in addition tothe detected temperature history, changes of the detected temperature Tdare also considered to estimate the amount of stored surplus heat usingthe second add/subtract value M2. Therefore, when the change of thedetected temperature Td is large (a change equal to or larger than thesecond add/subtract value temperature recovery temperature), since thesecond add/subtract value M2 has a non-negligible value, the amount ofstored surplus heat estimated value is modified, and the fuelutilization rate Uf is greatly changed.

Next, referring to FIGS. 29 through 32, we discuss limitations to thevariable range of generated power.

As described above, in the solid oxide fuel cell of the presentembodiment, the amount of surplus heat accumulated in the insulatingmaterial 7 or the like is utilized to increase the fuel utilizationrate, and the temperature inside the fuel cell module 2 is controlled toan appropriate temperature by actively utilizing the stored surplusheat. As explained with FIGS. 19 and 20, frequent increases anddecreases of the power produced by the fuel cell module 2 to meet thepower demand can cause the temperature inside the fuel cell module 2 torise excessively. It is possible to suppress such excessive temperaturerises by increasing the fuel utilization rate and actively utilizing theamount of surplus heat accumulated in the insulating material 7 or thelike. As explained using FIG. 24, however, in the range where the powergenerated is large, the minimum fuel utilization rate Ufmin takes alarge value Therefore, there is little room for increasing the fuelutilization rate and utilizing the stored surplus heat. Therefore, whenthe generated power is large, it is difficult to effectively reduce anexcessively raised temperature inside the fuel cell module 2 even byincreasing the fuel utilization rate and utilizing the stored surplusheat. For this reason, when an excessive temperature rise occurs in thefuel cell module 2 of the present embodiment, the range in which thegenerated power is variable to follow the power demand is restricted toa narrow range. Since this causes the fuel cell module 2 to be operatedto output small generated power, the margin for utilizing the storedsurplus heat increases, making it possible to effectively lower thetemperature inside the fuel cell module 2. By narrowing the range inwhich the generated power is variable to follow the power demand,temperature rises caused by frequent increases and decreases of thegenerated power are suppressed.

Note that temperature rises inside the fuel cell module 2 caused byfrequent increases and decreases of the power demand, as explained inFIGS. 19 and 20, also occur in the solid oxide fuel cell of the firstembodiment of the present invention described above. Therefore,referring to FIGS. 29 through 32, the limitations to the variable rangeof generated power explained below are explained, which can beimplemented in context of the above-described first embodiment of thepresent invention.

FIG. 29 is a flowchart showing a procedure for limiting the range of thepower produced by the fuel cell module of the present embodiment. FIG.30 is a map showing the limitations defined by the generated current andthe detected temperature Td. FIG. 31 is a timing chart showing anexemplary operation of the second embodiment of the present invention.FIG. 32 is a graph showing an example of the relationship between thetemperature inside the fuel cell module and the maximum generatablepower.

First, as shown by a solid line in FIG. 30, in the solid oxide fuel cellof the present embodiment, an appropriate temperature to be maintainedin the fuel cell module 2 is set for the generated current. Thisappropriate temperature corresponds to the dot-and-dash line in FIG. 22.As shown in FIG. 30, a current maintaining region is defined in theregion where the temperature is above the appropriate temperature. Theminimum temperature in the current maintaining region is defined todiffer according to the generated power from the fuel cell module 2, andthe current maintaining region minimum temperature is defined toincrease as the generated power increases. The minimum temperature inthe current maintaining region with respect to the generated power isdefined so that the difference relative to the appropriate temperaturefor the fuel cell module 2 increases as the generated power decreases.When the operating state of fuel cell module 2 transitions to thecurrent maintaining region, the output current from the fuel cell module2 is prohibited from increasing. Furthermore, a current reduction regionis defined in the region where the temperature is higher than thecurrent maintaining region. When the operating state transitions to thecurrent reduction region, the output current from the fuel cell module 2is forcibly reduced. An air cooling region is also defined in the regionwhere the temperature is above the current reduction region. When theoperating state transitions to the air cooling region, the generatingair supply rate is set to the maximum flow rate suppliable by theoxidant gas supply device 45. A stop operation region is also defined inthe region in which the temperature is higher than the air coolingregion. When the operating state transitions to the stop operationregion, the power generation by the fuel cell module 2 is stopped toprevent damage to the solid oxide fuel cell system.

Moreover, when the detected temperature Td has risen steeply, thetemperature line demarcating the current maintaining region is loweredas shown by the dot-and-dash line in FIG. 30. In such cases, thetemperature line demarcating the current reduction region is lowered asshown by the double dot-and-dash line in FIG. 30. Thus, when thedetected temperature Td steeply rises, a limitation is quickly imposedon the output current, thereby reliably preventing an excessivetemperature rise.

Next, referring to FIG. 29, we discuss the procedure for limitingcurrent produced by the fuel cell module.

First, the detected temperature Td is read in step S411 of FIG. 29.Next, in step S42, the detected temperature Td read in step S41 and apreviously detected temperature Td are compared. If the differencebetween the detected temperature Td read in step s41 and the previouslydetermined detected temperature Td is at or below a predeterminedthreshold temperature, the process advances to step S43.

In step S43, the map with the solid lines shown in FIG. 30 which definethe original locations of regions is selected as an initial map fordetermining a temperature region. If the difference between the latestdetected temperature Td and the previously determined detectedtemperature Td is larger than the predetermined threshold temperature,the process advances to S44 in which the solid lines are replaced withthe dot-and-dash lines and the double dot-and-dash lines shown in FIG.30 which are adopted when the temperature is steeply rising to revisethe original locations of the regions in the map for determining thetemperature region.

Next, in step S45, a judgment is made as to whether the detectedtemperature Td is within the stop operation region. In this embodiment,if the detected temperature Td is at or above 780° C., the temperatureis judged to be in the stop operation region. When it is judged that thedetected temperature Td is within the stop operation region, the processadvances to step S46. In step S46, the power generation by the fuel cellmodule 2 is stopped, and the solid oxide fuel cell system makes anemergency stop.

On the other hand, if it is judged in step S45 that the detectedtemperature Td is not within the stop operation region, the processadvances to step S47. In step S47 a judgment is made as to whether thedetected temperature Td is within the air cooling region. In the presentembodiment when the detected temperature Td is 750° C. or greater, it isjudged to be in the air cooling region. When it is judged that thedetected temperature Td is within the air cooling region, the processadvances to step S48.

In step S48, the generated current is fixed at a minimum current of 1 A,which will be consumed to run the auxiliary unit 4 without being outputto the inverter 54. The generating air supply rate is set to the maximumflow rate suppliable by the oxidant gas supply device 45. The watersupply rate is also increased and a ratio between the steam and carbonamounts is set to S/C=4. The process then completes one iteration of theprocess in the flow chart shown in FIG. 29.

On the other hand, if it is judged in step S47 that the detectedtemperature Td is not within the air cooling region, the processadvances to step S49. In step S49 a judgment is made as to whether thedetected temperature Td and the generated current are within the currentreduction region. If they are within the current reduction region, theprocess advances to step S50.

In step S50, the generated current from the fuel cell module 2 isforcibly set to 4 A or below. In other words, the upper limit value ofthe generated power from the fuel cell module 2 is reduced to atemperature rise-suppressing power (400 W), which is higher than ½ themaximum rated power of 700 W. Thereafter, when the power demanddeclines, the upper limit value of the generated power (current),following the power demand, is reduced, and the generated current ismaintained, not increased, even if the power demand grows. One iterationof the process in the flow chart of FIG. 29 is thus completed. This typeof limitation to the generated current continues until the detectedtemperature Td and the generated current go outside the currentreduction region.

On the other hand, if a judgment is made in step S49 that the detectedtemperature Td and the generated current are not within the currentreduction region, the process advances to step S51. In step S51 ajudgment is made as to whether the detected temperature Td and thegenerated current are within the current maintain region. If they arewithin the current reduction region, the process advances to step S52.

In step S52, the generated current is prohibited from increasing, andthereafter the generated current is maintained without being increased,even if the power demand grows. Subsequently, when the power demanddeclines, the upper limit value of the generated current (power),following the drop of the power demand, is reduced, and the upper limitof the generated current (power) is maintained, not raised, even if thepower demand grows. This type of limitation to the generated currentcontinues until the detected temperature Td and the generated currentmitigate outside the maintain current region, and the excessivetemperature rise of the fuel cell module 2 is resolved. One iteration ofthe process in the flow chart of FIG. 29 is thus completed.

In the present embodiment, restrictions on the generated power arestarted when the detected temperature Td rises above the temperatureline for defining the maintain current region for the generated current.The minimum temperature of the maintain current region relative to thegenerated current is referred to as a generated power restrictiontemperature (FIG. 30). The generated power restriction temperature isset higher in the present embodiment than the fuel utilization ratechange temperature (Ts(I)+Te) (FIG. 22) at which a change to increasethe fuel utilization rate is started.

On the other hand, if a judgment is made in step S51 that the detectedtemperature Td and the generated current are not within the maintaincurrent region, the process advances to step S53. At step S53,limitations are not imposed on the generated current, and controlutilizing the stored surplus heat is executed.

Next, referring to FIG. 31, we discuss an exemplary operation forlimiting the generated current.

The timing chart shown in FIG. 31 schematically depicts, in the orderfrom the top, changes of the detected temperature Td, a target current,the generated current, the fuel supply rate, the fuel utilization rate,and the air supply rate. The target current here refers to the currentdetermined from the power demand and the generated voltage.

First, at time t20 in FIG. 31, the generated current is approximately 6A, and the detected temperature Td is slightly below the appropriatetemperature at a generated current of 6 A (corresponding to t20 in FIG.30).

Next, at times t20-t21, because of larger increases and decreases of thepower demand repeated over a short period, the target current alsogreatly increases and decreases, and the generated current alsoincreases and decreases to follow the changes. By contrast, the fuelsupply rate, as explained in FIG. 20, is controlled to be held unchangedfor a predetermined time even after the generated current has declined,and controlled to be increased ahead of an increase of the generatedcurrent. Thus, the fuel supply rate becomes excessive relative to thegenerated current, resulting in that a large amount of excess fuel issupplied. Since the excess fuel is used to heat the interior of the fuelcell module 2, at times t20-t21, the detected temperature Td starts torise.

Furthermore, at time t21, the detected temperature Td reaches thetemperature boundary defining the maintain current region for holdingthe generated current at approximately 6 A (t21 in FIG. 30, correspondsthe transition from step S51→S52 in FIG. 29). Step S52 in FIG. 29 isthus executed in which the generated current is prohibited from rising,and the generated current is maintained. Therefore, at times t21-t22,the target current is growing to approximately 7 A, but the generatedcurrent is maintained at approximately 6 A. By prohibiting the generatedcurrent from rising, the upper limit value of the variable range of thegenerated power is reduced, and the variable range is narrowed, causingthe amount of excess fuel associated with changes of the power demand todecline. Thus, step S52 of FIG. 29, where the amount of excess fuelamount is reduced while electrical generation continues, is acting as atemperature rise suppressing circuit. Step S51 judges whether step S52,which acts as the temperature rise-suppressing circuit, is executed, andacts as an excess temperature rise estimating circuit for estimating anoccurrence of excessive temperature rises in the fuel cell module 2.

At times t21-t22, moreover, the detected temperature Td keeps rising.Therefore, the first add/subtract value M1 becomes a truly large value,and the first integrated value N1 id value also increases significantly.The fuel supply rate is thus reduced to increase the fuel utilizationrate Uf (FIG. 23). The increase of the fuel utilization rate Uf alsoacts to reduce the amount of excess fuel and lower the temperatureinside the fuel cell module, thus acting as a temperaturerise-suppressing circuit. Note that at times t21-t22, the fuelutilization rate Uf is increased, and the amount of surplus heataccumulated in the insulating material 7 or the like is activelyconsumed. However, because the thermal capacity of the fuel cell module2 is extremely large, the detected temperature Td continues to rise.

Next, at time t22, the increased fuel utilization rate Uf reaches themaximum fuel utilization rate Ufmax (=75%), which is the maximum fuelutilization rate achievable when a current of approximately 6 A isgenerated (first integrated value N1 id=1 in FIG. 23; FIG. 24). At timet22, the fuel utilization rate Uf is raised to the maximum fuelutilization rate Ufmax. Therefore, at times t22-t23, the fuelutilization rate Uf is maintained at the maximum fuel utilization rateUfmax. On the other hand, at times t22-t23, the detected temperature Tdis still continuing to rise. Therefore, the value of the secondintegrated value N2 id (having the same value as the first integratedvalue N1 id) also grows. Associated with this, the air utilization rateUa is reduced (N2 id in FIG. 25>1); i.e., the air supply rate isincreased.

In addition, at time t23, the detected temperature Td reaches thecurrent reduction region when a current of approximately 6 A isgenerated (corresponding to step S49→S50 in FIG. 29). This causes stepS50 in FIG. 29 to be executed, and the generated current is steeplyreduced from approximately 6 A to 4 A (t23→t23′ in FIG. 30), the upperlimit value of the generated power variable range is further reduced,and the variable range is further narrowed. Therefore, the fuelutilization rate Uf is reduced very slightly from the maximum fuelutilization rate Ufmax achievable when a current of 6 A is generated tothe maximum fuel utilization rate Ufmax when a current of 4 A isgenerated (FIGS. 24, 31). Note that at time t23, the fuel utilizationrate Uf is lowered. However, since the generated current is reduced to 4A, the absolute value of the fuel supply rate and the absolute amount ofthe excess fuel are lowered. Since the fuel utilization rate Uf ismaintained at the maximum fuel utilization rate Ufmax for the generatedcurrent being falling, consumption of the amount of accumulated surplusheat is further promoted. By reducing the generated current in this way,step S50 in FIG. 29, which reduces the amount of excess fuel whilegeneration continues, also acts as a temperature rise-suppressingcircuit. However, the detected temperature Td still rises at timest23-t24.

Next, at time t24, the detected temperature Td reaches the temperaturefor defining the air cooling region (step S47→S48 in FIG. 29,corresponding to t24 in FIG. 30). Step S48 in FIG. 29 is thus executed,and the air supply rate is increased to the maximum air supply rateachievable by the oxidant gas supply device 45. The generated current isgradually reduced from 4 A to 1 A. Thereafter, the generated currentreduced to 1 A, which is the temperature rise-suppressing generationamount, is maintained at the fixed level until the detected temperatureTd declines to a temperature below the temperature for defining thecurrent maintain region. The generated current which has been reduced to1 A is entirely consumed to operate the auxiliary unit 4 and not outputto the inverter 54. As the generated current drops, the fuel utilizationrate Uf is reduced from the maximum fuel utilization rate Ufmaxachievable when a current of 4 A is generated to the maximum fuelutilization rate Ufmax (=50%) achievable when a current of 1 A isgenerated (FIG. 24).

Thus, after step S50 in FIG. 29, which is the temperaturerise-suppressing circuit, is executed to stop the temperature fromrising to reduce the amount of excessive fuel, if further suppression oftemperature rise is required, the air supply is increased. The amount ofair increased beyond the supply rate needed for electrical generationacts as a cooling fluid flowing into the fuel cell module 2. Therefore,step S48 in FIG. 29 functions as a forced cooling circuit.

On the other hand, if by executing step S50, which is the temperaturerise-suppressing circuit for reducing the amount of excess fuel, thedetected temperature Td drops before reaching the temperature of the aircooling region, cooling by step S48, which is the forced coolingcircuit, is not executed. Therefore, a determination as to whether ornot to execute suppression of temperature rise by the forced coolingcircuit is made based on temperature changes in the fuel cell module 2after the temperature rise-suppressing circuit suppresses temperaturerises.

After time t24, the detected temperature Td continues to rise, but attime t25, the temperature shifts to decline (t24→t25 in FIG. 30).Thereafter, the detected temperature Td starts to decline, and at timet26 declines to reach the upper limit temperature of the currentreduction region (t25→t26 in FIG. 30). A reduction of the air supplyrate thus begins.

Next, at time t27, the temperature declines to the upper limittemperature of the maintain current region (t26→t27 in FIG. 30). Thedetected temperature Td continues to further decline, and at time t28declines to the lower limit temperature of the maintain current region(t27→t28 in FIG. 30).

At time t28, when the detected temperature Td goes down out of thecurrent maintain region, the generated current begins to increase inorder to meet the target current. In conjunction with this, the fuelsupply rate also starts to increase. While taking the value of themaximum fuel utilization rate Ufmax for the generated current, the fuelutilization rate Uf increases.

Note that in the above-described embodiment, temperature rises aresuppressed by lowering the upper limit of the variable range of thegenerated power according to the temperature inside the fuel cell module2. However, it is also possible as a variant example to suppresstemperature rises by lowering a frequency of increases and decreases ofthe generated power. That is, when the temperature inside the fuel cellmodule 2 has risen, a further temperature rise can be suppressed byreducing responsiveness to a rise of the power demand and reducing theresponsiveness to increase the generated power. When the responsivenessto an increase of the power demand is reduced, the generated powerincreases more slowly in response to increases of the power demand.Therefore, when the power demand increases and decreases frequently, therange where the generated power is variable to attempt to meet thechanges of the power demand becomes smaller, and the frequency at whichthe generated power increases and decreases is also reduced, so that theamount of excess fuel supplied accordingly declines. Therefore, areduction of the responsiveness to an increase of the power demandcontinues until an excessively high temperature inside the fuel cellmodule 2 is lowered.

Alternatively, a limitation can also be placed on the frequency at whichthe generated power is increased for a unit time to meet increases ofthe power demand. In this case, a number of times per predetermined timeunit may be used as a limitation at which the generated power starts torise. When the number of times per predetermined time becomes largerthan the limitation, the generated power is controlled so as not toallow the generated power to follow an increase of the power demand.

In the above-described embodiment, the generated current upper limit islowered to 4 A when the detected temperature Td reaches the currentreduction region. However, it is also possible, as a variant example, tomake variable the upper limit of the generated power. For example, thegenerated power upper limit value to be lowered is reduced more as thetemperature inside the fuel cell module 2 increases.

Next, referring to FIG. 32, we discuss the relationship between thetemperature inside the fuel cell module 2 and a maximum generatablepower.

As discussed above, there is a correlation between the generated power(current) from the fuel cell module 2 and the appropriate temperatureinside the fuel cell module 2, under which to obtain a large generatedpower requires raising the temperature inside the fuel cell module 2.However, when the fuel cell module 2 is in a temperature region over700° C., which is higher than the appropriate temperature relative tothe generated power, because of the characteristics of the individualfuel cell stack 14, the voltage generated by the fuel cell units 16declines. Therefore, when a large current is extracted from theindividual fuel cell stack 14 in order to output a large power, thetemperature of the individual fuel cell stack 14 further rises, and thegenerated voltage falls, resulting in that the output power does notincrease even though current is increased. As a result, where thetemperature in the fuel cell module 2 is high, the maximum poweractually generable declines as the temperature rises, as shown in FIG.32. When an attempt is made to extract the maximum rated power from thefuel cell module 2 in the temperature region, current is increased inorder to increase extracted power. A rise of current further increasesthe temperature of the fuel cell module 2 and reduces the powerextracted from the same. When such an attempt continues to obtain apredetermined rated power, a thermal runaway may occur which causes asudden temperature rise in the fuel cell module 2.

In the present embodiment, a thermal runaway can be prevented in advancein the region where the temperature inside the fuel cell module 2 ishigher than the appropriate temperature by maintaining or lowering thegenerated current even when the power demand has increased.

Next, referring to FIG. 33, we discuss measurement of the detectedtemperature Td in the present embodiment.

FIG. 33 is a flow chart showing a procedure for calculating a firstadd/subtract value M1 based on the temperatures Td detected by multipletemperature sensors.

As shown in FIG. 3, in the present embodiment, the two generatingchamber temperature sensors 142 are provided inside the generatingchamber 10. Inside the fuel cell module 2 in the present embodiment thetwenty fuel cell units 16 are arrayed in the width direction (FIG. 2)and eight fuel cell units 16 are arrayed in the depth direction (FIG.3). Therefore, a total of 160 fuel cell units 16 are arranged in arectangular fashion as seen in plan view. In this embodiment, of the twogenerating chamber temperature sensors 142, one is disposed adjacent tothe vertex of the rectangle, and the other is disposed adjacent to themidpoint of the long side of the rectangle. Thus, in the presentembodiment the two generating chamber temperature sensors 142 aredisposed so that different temperatures are detected within the fuelcell module 2.

Therefore, the temperature Td detected by the generating chambertemperature sensors 142 disposed adjacent to the rectangle verticesprimarily reflect the temperature of the fuel cell units 16 disposednear the vertices of the rectangle, and the temperature Td detected bythe generating chamber temperature sensors 142 disposed adjacent to themidpoint of a long side of the rectangle primarily reflect thetemperature of the fuel cell units 16 disposed near the midpoint of thelong side of the rectangle. The fuel cell units 16 disposed close to thevertices of the rectangle can easily be robbed of its heat by thesurrounding insulating material 7 or the like and are therefore at thelowest temperature. The fuel cell units 16 disposed near the midpoint ofthe long side of the rectangle reach a higher temperature than the fuelcell units 16 disposed near the vertices. In the present embodiment, thetemperature differences among the fuel cell units 16 may reach severaltens of degrees. Note that it is believed that the fuel cell units 16disposed near the intersection of the diagonals of the rectangle reachthe highest temperature, and the generating chamber temperature sensorsmay also be disposed so as to measure this temperature.

In step S61 of FIG. 33, the detected temperatures Td are read from thetwo generating chamber temperature sensors 142, respectively. Next, instep S62, an average value of the temperatures read from the detectedtemperature Td is calculated and a judgment is made as to whether theaveraged temperature is higher than the appropriate temperature Ts(I).When the averaged temperature is higher than the appropriate temperatureTs(I), the process advances to step S63. On the other hand, when theaveraged temperature is lower than the appropriate temperature Ts(I),the process advances to step S64.

In step S63, the first add/subtract value M1 is calculated based on thehigher of the two detected temperatures Td (the first add/subtract valueM1 becomes a positive value or 0) and the process for one iteration ofthe FIG. 33 flow chart is completed. That is, the estimated amount ofincrease of the stored surplus heat amount is determined based on thehigher of the two detected temperatures Td. In step S64, on the otherhand, the first add/subtract value M1 is calculated based on the lowerof the two detected temperatures Td (the first add/subtract value M1becomes a negative value or 0) and the process for one iteration of theFIG. 33 flow chart is completed. That is, the estimated amount ofreduction of the stored surplus heat amount is determined based on thelower of the two detected temperatures Td. Thus, the detectedtemperature Td on the higher temperature side is adopted when it isabove the appropriate temperature Ts(I), whereas the lower sidetemperature Td is adopted when it is below the appropriate temperatureTs(I). The amount of stored surplus heat is in this way estimated basedon the temperature of the higher temperature fuel cell unit 16 when anexcessive temperature rise becomes a problem. When a temperaturereduction is a problem, the amount of stored surplus heat is estimatedbased on the lower temperature among the fuel cell units 16 (normallythe fuel cell units positioned at vertices of the rectangle). Therefore,the amount of stored surplus heat can be estimated on the safe side evenwhen the temperature of each of the fuel cell units 16 differs.

Note that in the above-described embodiment, the detected temperature Tdon either the higher temperature side or the lower temperature side isselected and the integrated value is calculated based thereon. However,it is also acceptable as a variant example to obtain the integratedvalue from multiple detected temperatures Td. In the example, the amountof stored surplus heat is estimated by determining add/subtract valuesfor each of multiple detected temperatures and integrating thedetermined add/subtract values for the detected multiple temperatures tocalculate multiple integrated values. The largest numerical value of themultiple integrated values is selected when all of the multipleintegrated values are increasing, and the smallest numerical value ofthe multiple integrated values is selected when some of the multipleintegrated values are decreasing. The selected integrated value may beused to estimate the amount of stored surplus heat.

In the above-described present embodiment, the higher temperature sideof the detected temperature is adopted in step S63, and the lowertemperature side of the detected temperature is adopted in step S64.However, it is also possible as a variant example to calculate the firstadd/subtract value M1 based on a weighted average of the two detectedtemperatures to estimate the amount of stored surplus heat. For example,in step S63, the first add/subtract value M1 can be calculated based ona value obtained by adding the detected temperature on the highertemperature side multiplied by 0.7 and the detected temperature on thelower temperature side multiplied by 0.3. Then, in step S63, the firstadd/subtract value M1 can be calculated based on a value obtained byadding the detected temperature on the higher temperature sidemultiplied by 0.3 and the detected temperature on the lower temperatureside multiplied by 0.7. Thus, in step S63, where the detectedtemperature Td is high and the estimated amount of the stored surplusheat is increased (first add/subtract value M1 is positive or 0), thehighest of the multiple detected temperatures Td is used as the mostheavily weighted factor for estimating the amount of stored surplusheat, whereas in step S64, where the estimated amount of stored surplusheat is decreased (first add/subtract value M1 is negative or 0), thelowest temperature is used as the most heavily weighted factor forestimating the amount of stored surplus heat.

It is also possible to calculate the first add/subtract value M1 from asimple average of the detected temperatures Td, without weightingdetected temperatures Td.

When the temperature of fuel cell units positioned at the vertices ofthe rectangle has dropped to or below a predetermined usage-suppressingcell unit temperature, the first add/subtract value M1 can be determinedso as to suppress increases of the fuel utilization rate Uf.

Next, referring to FIG. 34, we discuss calculation of an add/subtractvalue according to a variant example of the present embodiment. Notethat calculation of the add/subtract value according to this variantexample may be used together with the process shown in FIG. 33, or maybe applied on its own. When the present variant example is applied onits own, it is acceptable to have one generating chamber temperaturesensor 142.

FIG. 34 is a flow chart showing the procedure for calculating a firstadd/subtract value M1 based on the detected temperature from thereformer temperature sensor 148, which is another temperature detectiondevice provided in addition to the generating chamber temperaturesensors 142 serving as temperature detection device.

First, in step S71 of FIG. 34, the detected temperatures are read fromthe reformer temperature sensors 148. In the present embodiment, thereare reformer temperature sensors 148 attached at two locations, on theentrance side of the reformer 20 and on the exit side of the reformer20, so that temperatures in the vicinity of the entrance and exit of thereformer 20 are measured. Normally the temperature of the reformer 20 islow on the entrance side where the endothermic steam reforming reactionoccurs abundantly, and high on the exit side.

Next, in step S72 each of the detected temperatures in the reformer 20is compared to a predetermined usage-suppressing reformer temperature.First, if the lower of the two temperatures detected at the reformer 20is lower than a low temperature-side usage-suppressing reformertemperature Tr0, and the higher of the detected temperatures is lowerthan a high temperature-side usage-suppressing reformer temperature Tr1,the process advances to step S73. On the other hand, if the higher ofthe two temperatures detected at the reformer 20 is higher than the hightemperature-side usage-suppressing reformer temperature Tr1, and thelower of the detected temperatures is higher than the lowtemperature-side usage-suppressing reformer temperature Tr0, the processadvances to step S75. When neither of these is true, the processadvances to step S74.

In step S73, because the temperature of the reformer 20 is lower thaneach of the usage-suppressing reformer temperatures, the firstadd/subtract value M1 is corrected so that the fuel utilization rate Uffalls (the fuel supply rate increases). That is, a value obtained bysubtracting 10% of the absolute value of first add/subtract value M1calculated based on the temperature Td detected by the generatingchamber temperature sensors 142 is used for integration. The firstintegrated value N1 id, which is the estimated amount of stored surplusheat, thus decreases (its increase is suppressed), placing the fuelutilization rate Uf to decrease (an increase of the fuel utilizationrate is suppressed), thereby raising the temperature of the reformer 20.

In step S75, on the other hand, because the temperature of the reformer20 is higher than each of the usage-suppressing reformer temperatures,the first add/subtract value M1 is corrected so that the fuelutilization rate Uf rises (the fuel supply rate decreases). That is, avalue obtained by adding, to the first add/subtract value M1, 10% of theabsolute value of first add/subtract value M1 calculated based on thetemperature Td detected by generating chamber temperature sensors 142 isused as the first add/subtract value M1 for integration. The firstintegrated value N1 id, which is the estimated amount of stored surplusheat, thus increases (its decrease is suppressed), causing the fuelutilization rate Uf to rise, so that the temperature of the reformer 20is decreased. Damage to the reformer 20 caused by an excessive rise ofthe temperature in the reformer 20 is thus prevented.

In step S74, because the reformer 20 is in the appropriate temperaturerange, no correction is made to the first add/subtract value M1, and theprocess of one iteration of the FIG. 34 flow chart is completed. (Sincethe two temperatures detected at the reformer 20 are correlated, it doesnot normally happen that the lower detected temperature is lower thanlow temperature-side usage-suppressing reformer temperature, and thehigher of the detected temperatures is higher than high temperature-sideusage-suppressing reformer temperature.)

Note that in the variant example of present embodiment the fuelutilization rate may also be corrected by averaging the temperaturesdetected by the two reformer temperature sensors 148 and comparing theaveraged detected temperature to one or two of the usage-suppressingreformer temperatures. It is also possible to use a change rate of thetemperatures detected by the reformer temperature sensors 148. When thechange rate is high, the amount of correction to the fuel utilizationrate may be increased.

Next, referring to FIG. 35, we discuss calculation of an add/subtractvalue according to a variant example of the present embodiment. Notethat calculation of the add/subtract value according to this variantexample may be used together with the process in FIGS. 33 and 34, or maybe applied on its own. When the present variant example is applied onits own, it is acceptable to use one generating chamber temperaturesensor 142.

FIG. 35 is a flow chart showing the procedure for calculating a firstadd/subtract value M1 based on the detected temperature from an exhausttemperature sensor 140, which is another temperature detection device inaddition to the generating chamber temperature sensors 142 serving astemperature detection device.

First, in step S81 of FIG. 35, detected temperatures are read from theexhaust temperature sensor 140. In the present embodiment, the exhausttemperature sensor 140 is disposed to measure the temperature of exhaustgas combusted in the combustion chamber 18 and discharged after passingthrough an exhaust gas discharge pipe 82.

Next, in step S82, the detected temperature of the exhaust gas iscompared to a predetermined usage-suppressing exhaust gas temperature.First, if the detected exhaust gas temperature is below a predeterminedlow temperature side usage-suppressing exhaust temperature Tem0, theprocess advances to step S83. On the other hand, if the detected exhaustgas temperature is above a predetermined high temperature sideusage-suppressing exhaust temperature Tem1, the process advances to stepS85. If the exhaust gas detected temperature is below the hightemperature side usage-suppressing exhaust temperature Tem1 and abovethe low temperature side usage-suppressing exhaust temperature Tem0, theprocess advances to step S84.

In step S83, since the temperature of the exhaust gas is lower than theappropriate temperature, the first add/subtract value M1 is corrected sothat the fuel utilization rate Uf decreases (the fuel supply rateincreases). That is, a value obtained by subtracting from the firstadd/subtract value M1 10% of the absolute value of first add/subtractvalue M1 calculated based on the temperature Td detected by generatingchamber temperature sensors 142 is used as the first add/subtract valueM1 for integration. The first integrated value N1 id, which is theestimated amount of stored surplus heat, thus decreases (its increase issuppressed), causing the fuel utilization rate Uf to decrease (anincrease of the fuel utilization rate Uf is suppressed), so that thetemperature of the exhaust gas is raised.

In step S85, on the other hand, since the temperature of the exhaust gasis higher than the appropriate temperature, the first add/subtract valueM1 is corrected so that the fuel utilization rate Uf increases (the fuelsupply rate decreases). That is, a value obtained by adding to the firstadd/subtract value M1 10% of the absolute value of first add/subtractvalue M1 calculated based on the temperature Td detected by generatingchamber temperature sensors 142 is used as the first add/subtract valueM1 for integration. The first integrated value N1 id, which is theestimated amount of stored surplus heat, thus increases (its decrease issuppressed), causing the fuel utilization rate Uf to rise, so that thetemperature of the exhaust gas is decreased. The temperature inside thefuel cell module 2 is by this means controlled to become the appropriatetemperature.

In step S84, because the exhaust gas is in the appropriate temperaturerange, no correction is made to the first add/subtract value M1, andprocess for one iteration of the FIG. 35 flow chart is completed.

Note that this variant example may adopt a correction based on the rateof change per unit time of the temperatures detected by the exhausttemperature sensor 140. This is, when the rate of change is high, therange of correcting the fuel utilization rate may be increased.

In the solid oxide fuel cell according to the second embodiment of thepresent invention, the amount of stored surplus heat is estimated basedon the temperature Td detected by the generating chamber temperaturesensors 142. Therefore, the control section 110 can accurately estimatethe amount of accumulated surplus heat even if the output power ischanged with a delay time (FIG. 20) after the fuel supply rate ischanged. It is possible that the amount of surplus heat stored in theheat storing material 2 can be fully utilized while the risk of a suddentemperature drop in the fuel cell module 2 can be securely avoided. Inaddition, in fuel cells of the type, as used in the present embodiment,in which the output power is changed with a delay time after the fuelsupply rate is changed, frequent increase and decrease of the outputpower cause a large amount of excess fuel and pose the risk of raisingthe temperature excessively inside the fuel cell module (FIG. 31).However, in the solid oxide fuel cell of the present embodiment, theamount of stored surplus heat caused by the excess fuel can beaccurately estimated. In the prior art, a cooling medium (air) isintroduced in the fuel cell module 2 to suppress an excessivetemperature rise caused by excess fuel. However, in the solid oxide fuelcell of present embodiment, since the amount of surplus heat caused byexcess fuel is accurately estimated, by effectively using the storedsurplus heat, an excessive temperature rise can be suppressed. An extraamount of generating air that would need to be introduced in to lowerthe temperature can be dispensed with, and the overall energy efficiencyof the solid oxide fuel cell can be improved.

In the solid oxide fuel cell of the present embodiment, the stored heatamount estimating circuit 110 b estimates the amount of stored surplusheat based on the history of detected temperatures Td (first integratedvalue N1 id). Therefore, the amount of the stored surplus heat can bemore accurately estimated than it is when only a recently detectedtemperature Td is used. The amount of surplus heat stored in the heatstoring material can therefore be fully utilized.

Furthermore, in the present invention thus constituted, a large amountof the stored surplus heat is used when the estimated amount of storedsurplus heat is large, and a lesser amount of the stored surplus heat isused when the amount of stored surplus heat is estimated small (FIG.23). Therefore, the stored surplus heat can be more effectivelyutilized, and the risk of a temperature drop can be reliably avoided.

According to the solid oxide fuel cell of the present embodiment, sinceconditions such as degradation of the fuel cell module 2 (FIG. 28), inaddition to the amount of stored surplus heat and the power demand, areused to determine the fuel utilization rate, the stored surplus heat canbe appropriately utilized according to the state of the fuel cell module2.

In addition, in the solid oxide fuel cell of the present embodiment, thefuel utilization rate is changed more in the range where the amount ofstored surplus heat estimated by the stored heat amount estimatingcircuit 110 b is large (FIG. 23, N1 id=around 0.9 to 1) than it is inthe range where the estimated stored heat amount is small (N1 id=around0 to 0.1). Therefore, when the estimated amount of the stored surplusheat is large, a large amount of stored surplus heat is utilized and anexcessive temperature rise can be avoided, whereas when the estimatedamount of stored surplus heat is small, a lesser amount of the storedsurplus heat is used, and overcooling can be avoided.

In the solid oxide fuel cell of the present embodiment, the storedsurplus heat is estimated by taking into consideration not only thehistory of the detected temperatures Td but also changes of recentlydetected temperature (second add/subtract value M2). Therefore, thestored surplus heat can be accurately estimated based on the history,and the responsiveness can be improved to temperatures changes of thefuel cell module 2, which has a large thermal capacity and thus isdifficult to change a direction of temperature change, therebypreventing excessive temperature rises and temperature drops.

Furthermore, in the solid oxide fuel cell of the present embodiment, thefuel utilization rate is changed in a wider range where the generatedpower is small than where the generated power is large (FIG. 24). Therisk of a sudden temperature drop can therefore be reduced, and theenergy efficiency can be effectively increased by utilizing a largeamount of stored surplus heat where the generated power is small (aroundIU in FIG. 24) and therefore there is a greater margin for improving thefuel utilization rate. In the range where the generated power is large,and thus the fuel utilization rate is normally high (around the regionclose to Imax in FIG. 24), only a little improvement is possible on thefuel utilization rate. During the operation in the range, the storedsurplus heat is less used, and the surplus heat accumulated during theoperation in the range is used where the generated power is smallwhereby the energy efficiency where the generated power is small can beimproved.

In the solid oxide fuel cell of the present embodiment, a range ofchanges causing the fuel utilization rate to rise is reduced(integration of N1 id is stopped) after the fuel cell module hasdegraded (FIG. 28). Therefore, by operating the degraded fuel cellmodule, in which the operation temperature tends to be higher, in alower temperature range, degradation can be prevented from advancing.

Furthermore, in the solid oxide fuel cell of the present embodiment, theamount of stored surplus heat is estimated based on integration of theadd/subtract values (N1 id) and on the differential value (M2).Therefore, the amount of stored surplus heat can be appropriatelyestimated by a simple calculation, whereby the fuel utilization rate canbe set appropriately.

Also, in the solid oxide fuel cell of the present embodiment, since theadd/subtract values (M1, M2) for use in estimating the amount of storedsurplus heat are determined based on the generated power, the generatedcurrent, and other conditions in addition to the detected temperatureTd, factors other than the temperature affecting the stored surplus heatcan be appropriately taken into consideration when the estimated valueis calculated.

In addition, in the solid oxide fuel cell of the present embodiment,since an increase or decrease of the amount of stored surplus heat isestimated not only with the detected temperature Td but also with thegenerated power (Equations (5), (7), etc.), a more accurate estimationof the stored surplus heat becomes possible.

Furthermore, in the solid oxide fuel cell of the present embodiment, theestimated amount of stored surplus heat changes more quickly as thegenerated power increases (the flow chart in FIG. 21 is executed inshorter intervals as the generated power increases). Therefore, it ispossible to estimate the amount of stored surplus hear more accurately.

The preferred embodiments of the present invention have explained above.However, various modifications to the above-described embodiments may bepossible. In particular, in the above-described embodiments, the heatcapacity of the insulation material (heat storing material) is fixed.However, as a variant example, a fuel cell module with a variable heatcapacity may be used with the present invention. In that case, anadditional heat capacity member with a large heat capacity can beprovided so as to be able to be thermally connected to and removed fromthe fuel cell module. If the heat capacity should be large, anadditional heat capacity member may be thermally connected to the fuelcell module, whereas if the heat capacity should be small, an additionalheat capacity member may be thermally removed. For example, during thestart-up of the solid oxide fuel cell, the heat capacity is made smallby removing the additional heat capacity member, thereby raising thetemperature of the fuel cell module. On the other hand, when it isanticipated that the solid oxide fuel cell is operated to output largegenerated power for long hours, the fuel cell module is connected withan additional heat capacity member so as to accumulate a larger surplusheat amount.

What is claimed is:
 1. A solid oxide fuel cell system operable togenerate electrical power variable in response to power demand,comprising: a fuel cell module configured to generate the electricalpower using fuel; a fuel supply device configured to supply the fuel ata fuel supply rate to the fuel cell module; an oxidant gas supply deviceconfigured to supply to the fuel cell module oxidant gas at an oxidantgas supply rate for generation of the electrical power; a combustionsection configured to burn excess fuel from the fuel supply device whichis left unused for generation of electrical power and heat an interiorof the fuel cell module; a heat storing material configured to storeheat generated within the fuel cell module; a power demand detectiondevice configured to detect the power demand; a temperature detectiondevice configured to detect a temperature of the fuel cell module; and acontrol device programmed to operate the fuel supply device, responsiveto a change of the power demand detected by the power demand detectiondevice, to change the fuel supply rate to change the electrical poweroutput from the fuel cell in order to meet the change of the powerdemand, wherein the control device is programmed to place a delay timebefore the electrical power output from the fuel cell module is changedafter the fuel supply rate is changed in response to the change of thepower demand, wherein the control device includes a stored heatestimating circuit configured to estimate, from a temperature detectedby the temperature detection device, an amount of surplus heat stored inthe heat storing material, and in response to an estimation by thestored heat estimating circuit that a first amount of surplus heat isstored in the heat storing material, the control device is programmed tocontrol the fuel supply rate to output a level of electrical power at afirst fuel utilization rate, and in response to an estimation by thestored heat estimating circuit that a second amount of surplus heat,which is higher than the first amount of surplus heat, is stored in theheat storing material, control the fuel supply rate to output the samelevel of electrical at a second fuel utilization rate, which is higherthan the first fuel utilization rate, wherein the fuel utilization rateis a rate of an amount of fuel used for generation of the electricalpower with respect to a total amount of fuel supplied to the fuel cellmodule.
 2. The solid oxide fuel cell according to claim 1, wherein thestored heat estimating circuit is configured to estimate the amount ofsurplus heat stored in the heat storing material based on a history oftemperatures detected by the temperature detection device.
 3. The solidoxide fuel cell according to claim 2, wherein the control device isprogrammed to operate the fuel supply device to increase the fuelutilization rate as the amount of stored surplus heat estimated by thestored heat estimating circuit increases.
 4. The solid oxide fuel cellaccording to claim 1, wherein the control device is programmed tooperate the fuel supply device in response to the power demand toachieve a third fuel utilization rate when the electrical power isoutput from the fuel cell module at a first level and achieve a fourthfuel utilization rate, which is lower than the third fuel utilizationrate, when the electrical power is output from the fuel cell mode at asecond level which is lower than the first level of electrical power. 5.The solid oxide fuel cell according to claim 3, wherein the controldevice is programmed to operate the fuel supply device to increase thefuel utilization rate to an increasing degree as the amount of storedsurplus heat estimated by the stored heat estimating circuit increases.6. The solid oxide fuel cell according to claim 2, wherein the storedheat estimating circuit is configured to additionally use a degree towhich the temperature detected by the temperature detection device isrising to account for estimation of the amount of stored surplus heat sothat the estimated amount of surplus heat increases as the degree towhich the detected temperature is rising increases.
 7. The solid oxidefuel cell according to claim 1, wherein the control device is programmedto change the fuel utilization rate according to the estimated amount ofstored surplus heat between a maximum fuel utilization rate and aminimum fuel utilization rate, a difference between which decreases asthe generated electrical power increases.
 8. The solid oxide fuel cellaccording to claim 1, wherein the control device is programmed tooperate the fuel supply device to change the fuel utilization ratewithin a first range before the fuel cell module degrades and change thefuel utilization rate within a second range, which is narrower than thefirst range, after the fuel cell module has degraded.
 9. The solid oxidefuel cell according to claim 1, wherein when estimating the amount ofstored surplus heat, the stored heat estimating circuit is configured toadd or subtract add/subtract values that comprise a first group ofvalues assigned to temperatures detected by the temperature detectiondevice and a second group of values added or subtracted when thedetected temperature is rising or falling.
 10. The solid oxide fuel cellaccording to claim 1, wherein when estimating the amount of storedsurplus heat, the stored heat estimating circuit is configured to addand subtract a set of add/subtract values that comprises a first valuethat is a function of a temperature detected by the temperaturedetection device and a second value reflecting rising or falling of thedetected temperature.
 11. The solid oxide fuel cell according to claim10, wherein the first value takes a positive or negative value dependingon a degree of the detected temperature relative to a thresholdtemperature predicted from the generated electrical power.
 12. The solidoxide fuel cell according to claim 11, wherein the stored heatestimating circuit is configured to estimate the amount of storedsurplus heat at intervals that become shorter as the generatedelectrical power increases.
 13. The solid oxide fuel cell according toclaim 10, wherein when estimating the amount of stored surplus heat, thestored heat estimating circuit is configured to recursively add andsubtract the set of add/subtract values.
 14. The solid oxide fuel cellaccording to claim 1, further comprising a plurality of temperaturedetection devices, including the temperature detection device,configured to detect temperatures of the fuel cell module at multipleplaces of the fuel cell module, wherein the stored heat estimatingcircuit is configured to estimate the amount of surplus heat stored inthe heat storing material from histories of temperatures detected by theplurality of temperature detection devices.
 15. The solid oxide fuelcell according to claim 14, wherein when estimating the amount of storedsurplus heat, the stored heat estimating circuit is configured tocalculate a weighted average of the detected temperatures in such amanner that a highest temperature among the detected temperatures isweighted most when the fuel cell module is heated above a thresholdtemperature predicted from the generated electrical power, and a lowesttemperature among the detected temperatures is weighted most when thefuel cell module is heated below the threshold temperature.
 16. Thesolid oxide fuel cell according to claim 14, wherein the plurality oftemperature detection devices are disposed to detect a temperature ofthe reformer and a temperature of the fuel cell stack, and when thedetected temperature of the reformer is below a predetermined thresholdtemperature, the stored heat estimating circuit is configured toestimate the amount of stored surplus heat lower than it does when thedetected temperature of the reformer is equal to or above thepredetermined threshold temperature.
 17. The solid oxide fuel cellaccording to claim 14, wherein the fuel cell stack comprises multiplefuel cell units arrayed in approximately a rectangular form, one of theplurality of temperature detection devices is disposed to detect atemperature of a fuel cell unit positioned adjacent to a vertex of therectangle, another one of the multiple temperature detection devices isdisposed to detect a temperature of a fuel cell unit positioned adjacentto a center between two vertices of the rectangle, and when the detectedtemperature of the fuel cell unit positioned at the vertex of therectangle is below a predetermined threshold temperature, the storedheat estimating circuit is configured to estimate the amount of storedsurplus heat lower than it does when the detected temperature of thefuel cell unit positioned at the vertex of the rectangle is equal to orabove the predetermined threshold temperature.